Patterning process, resist composition, and acetal compound

ABSTRACT

A pattern is formed by applying a resist composition comprising a polymer comprising recurring units having an acid labile group-substituted hydroxyl group, an acid generator, and an organic solvent onto a substrate, prebaking the composition to form a resist film, exposing the resist film to high-energy radiation to define exposed and unexposed regions, baking, and developing the exposed film with an organic solvent developer to form a negative pattern wherein the unexposed region of film is dissolved and the exposed region of film is not dissolved.

CROSS-REFERENCE TO RELATED APPLICATION

This non-provisional application claims priority under 35 U.S.C. §119(a)on Patent Application No. 2010-067754 filed in Japan on Mar. 24, 2010,the entire contents of which are hereby incorporated by reference.

TECHNICAL FIELD

This invention relates to a pattern forming process involving exposureof resist film, deprotection reaction with the aid of acid and heat, anddevelopment with an organic solvent to form a negative tone pattern inwhich the unexposed region is dissolved and the exposed region is notdissolved, a resist composition for use in the process, and an acetalcompound.

BACKGROUND ART

In the recent drive for higher integration and operating speeds in LSIdevices, the pattern rule is made drastically finer. Thephotolithography which is currently on widespread use in the art isapproaching the essential limit of resolution determined by thewavelength of a light source. As the light source used in thelithography for resist pattern formation, g-line (436 nm) or i-line (365nm) from a mercury lamp was widely used in 1980's. Reducing thewavelength of exposure light was believed effective as the means forfurther reducing the feature size. For the mass production process of 64MB dynamic random access memories (DRAM, processing feature size 0.25 μmor less) in 1990's and later ones, the exposure light source of i-line(365 nm) was replaced by a KrF excimer laser having a shorter wavelengthof 248 nm. However, for the fabrication of DRAM with a degree ofintegration of 256 MB and 1 GB or more requiring a finer patterningtechnology (processing feature size 0.2 μm or less), a shorterwavelength light source was required. Over a decade, photolithographyusing ArF excimer laser light (193 nm) has been under activeinvestigation. It was expected at the initial that the ArF lithographywould be applied to the fabrication of 180-nm node devices. However, theKrF excimer lithography survived to the mass-scale fabrication of 130-nmnode devices. So, the full application of ArF lithography started fromthe 90-nm node. The ArF lithography combined with a lens having anincreased numerical aperture (NA) of 0.9 is considered to comply with65-nm node devices. For the next 45-nm node devices which required anadvancement to reduce the wavelength of exposure light, the F₂lithography of 157 nm wavelength became a candidate. However, for thereasons that the projection lens uses a large amount of expensive CaF₂single crystal, the scanner thus becomes expensive, hard pellicles areintroduced due to the extremely low durability of soft pellicles, theoptical system must be accordingly altered, and the etch resistance ofresist is low; the development of F₂ lithography was abandoned andinstead, the ArF immersion lithography was introduced.

In the ArF immersion lithography, the space between the projection lensand the wafer is filled with water having a refractive index of 1.44.The partial fill system is compliant with high-speed scanning and whencombined with a lens having a NA of 1.3, enables mass production of45-nm node devices.

One candidate for the 32-nm node lithography is lithography usingextreme ultraviolet (EUV) radiation with wavelength 13.5 nm. The EUVlithography has many accumulative problems to be overcome, includingincreased laser output, increased sensitivity, increased resolution andminimized line edge or width roughness (LER, LWR) of resist film,defect-free MoSi laminate mask, reduced aberration of reflection mirror,and the like.

Another candidate for the 32-nm node lithography is high refractiveindex liquid immersion lithography. The development of this technologywas abandoned because LUAG, a high refractive index lens candidate had alow transmittance and the refractive index of liquid did not reach thegoal of 1.8.

The process that now draws attention under the above-discussedcircumstances is a double patterning process involving a first set ofexposure and development to form a first pattern and a second set ofexposure and development to form a pattern between the first patternfeatures. A number of double patterning processes are proposed. Oneexemplary process involves a first set of exposure and development toform a photoresist pattern having lines and spaces at intervals of 1:3,processing the underlying layer of hard mask by dry etching, applyinganother layer of hard mask thereon, a second set of exposure anddevelopment of a photoresist film to form a line pattern in the spacesof the first exposure, and processing the hard mask by dry etching,thereby forming a line-and-space pattern at a half pitch of the firstpattern. An alternative process involves a first set of exposure anddevelopment to form a photoresist pattern having spaces and lines atintervals of 1:3, processing the underlying layer of hard mask by dryetching, applying a photoresist layer thereon, a second set of exposureand development to form a second space pattern on the remaining hardmask portion, and processing the hard mask by dry etching. In eitherprocess, the hard mask is processed by two dry etchings.

As compared with the line pattern, the hole pattern is difficult toreduce the feature size. In order for the prior art method to form fineholes, an attempt is made to form fine holes by under-exposure of apositive resist film combined with a hole pattern mask. This, however,results in the exposure margin being extremely narrowed. It is thenproposed to form holes of greater size, followed by thermal flow orRELACS® method to shrink the holes as developed. However, there is aproblem that control accuracy becomes lower as the pattern size afterdevelopment and the size after shrinkage differ greater and the quantityof shrinkage is greater. With the hole shrinking method, the hole sizecan be shrunk, but the pitch cannot be narrowed.

It is then proposed in Proc. SPIE, Vol. 5377, p. 255 (2004) that apattern of X-direction lines is formed in a positive resist film usingdipole illumination, the resist pattern is cured, another resistmaterial is coated thereon, and a pattern of Y-direction lines is formedin the other resist film using dipole illumination, leaving alattice-like line pattern, interstices of which provide a hole pattern.Although a hole pattern can be formed at a wide margin by combining Xand Y lines and using dipole illumination featuring a high contrast, itis difficult to etch vertically staged line patterns at a highdimensional accuracy. It is proposed in IEEE IEDM Tech. Digest 61 (1996)to form a hole pattern by exposure of a negative resist film through aLevenson phase shift mask of X-direction lines combined with a Levensonphase shift mask of Y-direction lines. However, the crosslinkingnegative resist film has the drawback that the resolving power is low ascompared with the positive resist film, because the maximum resolutionof ultrafine holes is determined by the bridge margin.

A hole pattern resulting from a combination of two exposures of X- andY-direction lines and subsequent image reversal into a negative patterncan be formed using a high-contrast line pattern of light. Thus holeshaving a narrow pitch and fine size can be opened as compared with theprior art.

Proc. SPIE Vol. 7274, p. 72740N (2009) reports three methods for forminghole patterns via image reversal. The three methods are: method (1)involving subjecting a positive resist composition to two double-dipoleexposures of X and Y lines to form a dot pattern, depositing a SiO₂ filmthereon by LPCVD, and effecting O₂-RIE for reversal of dots into holes;method (2) involving forming a dot pattern by the same steps as in (1),but using a resist composition designed to turn alkali-soluble andsolvent-insoluble upon heating, coating a phenol-base overcoat filmthereon, effecting alkaline development for image reversal to form ahole pattern; and method (3) involving double dipole exposure of apositive resist composition and organic solvent development for imagereversal to form holes.

The formation of negative pattern through organic solvent development isa traditional technique. A resist composition comprising cyclized rubberis developed using an alkene such as xylene as the developer. An earlychemically amplified resist composition comprisingpoly(t-butoxycarbonyloxystyrene) is developed with anisole as thedeveloper to form a negative pattern.

Recently a highlight is put on the organic solvent development again. Itwould be desirable if a very fine hole pattern, which is not achievablewith the positive tone, is resolvable through negative tone exposure. Tothis end, a positive resist composition featuring a high resolution issubjected to organic solvent development to form a negative pattern. Anattempt to double a resolution by combining two developments, alkalidevelopment and organic solvent development is under study.

As the ArF resist composition for negative tone development with organicsolvent, positive ArF resist compositions of the prior art design may beused. Pattern forming processes are described in JP-A 2008-281974, JP-A2008-281975, and JP 4554665.

These patent documents disclose resist compositions for organic solventdevelopment comprising a copolymer of hydroxyadamantane methacrylate, acopolymer of norbornane lactone methacrylate, and a copolymer ofmethacrylate having acidic groups including carboxyl, sulfo, phenol,thiol and other groups substituted with two or more acid labile groups,and pattern forming processes using the same.

Further, JP-A 2008-309878 discloses a process for forming a patternthrough organic solvent development in which a protective film isapplied onto a resist film. JP-A 2008-309879 discloses a topcoatlessprocess for forming a pattern through organic solvent development inwhich an additive is added to a resist composition so that the additivemay segregate at the resist film surface after spin coating to providethe surface with improved water repellency.

CITATION LIST

-   Patent Document 1: JP-A 2008-281974-   Patent Document 2: JP-A 2008-281975-   Patent Document 3: JP 4554665-   Patent Document 4: JP-A 2008-309878-   Patent Document 5: JP-A 2008-309879-   Non-Patent Document 1: Proc. SPIE Vol. 5377, p. 255 (2004)-   Non-Patent Document 2: IEEE IEDM Tech. Digest 61 (1996)-   Non-Patent Document 3: Proc. SPIE Vol. 7274, p. 72740N (2009)

DISCLOSURE OF INVENTION

As compared with the positive resist system which becomes dissolvable inalkaline developer as a result of acidic carboxyl or analogous groupsgenerating through deprotection reaction, the organic solventdevelopment provides a low dissolution contrast. The alkaline developerprovides an alkaline dissolution rate that differs by a factor of 1,000or more between the unexposed and exposed regions whereas the organicsolvent development provides a dissolution rate difference of only about10 times. While Patent Documents 1 to 6 describe conventionalphotoresist compositions of the alkaline aqueous solution developmenttype, there is a demand for a novel material which can offer asignificant dissolution contrast upon organic solvent development.

When holes are formed by negative development, regions surrounding theholes receive light so that excess acid is generated therein. It is thenimportant to control acid diffusion because the holes are not opened ifthe acid diffuses inside the holes.

If the acid in the exposed region evaporates during PEB and deposits onthe unexposed region, the positive pattern following alkalinedevelopment suffers from such drawbacks as rounded top of its profileand film slimming. An inverse phenomenon occurs on negative developmentwith organic solvent, that is, holes are not opened or the opening sizeof holes at the top is reduced.

Coverage of a photoresist film with a protective film is effective forpreventing evaporation of acid during PEB and for avoiding any holeopening failure following negative development, but still insufficient.The problem of hole opening failure following negative development isserious if a photoresist film is not covered with a protective film.

An object of the invention is to provide a photoresist composition whichestablishes a high sensitivity and a high dissolution contrast duringorganic solvent development, a pattern forming process for forming ahole pattern via positive/negative reversal using a mask bearing alattice-like pattern for forming a hole pattern, and an acetal compound.

The inventors have found that a resist composition comprising a polymercomprising recurring units having an acid labile group-substitutedhydroxyl group is improved in dissolution contrast during organicsolvent development and forms a hole pattern via positive/negativereversal which is improved in sensitivity, resolution, and dimensionaluniformity.

In one aspect, the invention provides a pattern forming processcomprising the steps of applying a resist composition comprising apolymer comprising recurring units having an acid labilegroup-substituted hydroxyl group, an acid generator, and an organicsolvent onto a substrate, heat treating the composition to form a resistfilm, exposing the resist film to high-energy radiation to defineexposed and unexposed regions, heat treating, and developing the exposedfilm with an organic solvent developer to form a negative patternwherein the unexposed region of film is dissolved and the exposed regionof film is not dissolved.

In a preferred embodiment, the polymer comprises recurring units (a1)and/or (a2) represented by the general formula (1).

In formula (1), R¹ and R⁴ are each independently hydrogen or methyl, R²is a straight, branched or cyclic, di- to penta-valent aliphatichydrocarbon group of 1 to 16 carbon atoms which may contain an ether orester radical, R³ and R⁵ each are an acid labile group, with the provisothat when R² contains an adamantane ring, an acid labile group of acetalform having the general formula (2):

wherein R⁶ is a straight, branched or cyclic, monovalent hydrocarbongroup of 1 to 10 carbon atoms, R⁷ and R⁸ are each independently hydrogenor a straight, branched or cyclic, monovalent hydrocarbon group of 1 to10 carbon atoms, R⁷ and R⁸ may bond together to form an aliphatichydrocarbon ring with the carbon atom to which they are attached, and R⁹is a straight, branched or cyclic, monovalent hydrocarbon group of 1 to15 carbon atoms is excluded from R³, m is an integer of 1 to 4, a1 anda2 are numbers in the range: 0≦a1<1.0, 0≦a2<1.0, and 0<a1+a2<1.0.

In a preferred embodiment, the developer comprises at least one solventselected from the group consisting of 2-octanone, 2-nonanone,2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone,diisobutyl ketone, methylcyclohexanone, acetophenone,methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate,amyl acetate, butenyl acetate, isoamyl acetate, phenyl acetate, propylformate, butyl formate, isobutyl formate, amyl formate, isoamyl formate,methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate,methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyllactate, amyl lactate, isoamyl lactate, methyl 2-hydroxyisobutyrate,ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenylacetate, benzyl acetate, methyl phenylacetate, benzyl formate,phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate.

In a preferred embodiment, the step of exposing the resist film tohigh-energy radiation includes ArF immersion lithography of 193 nmwavelength or EUV lithography of 13.5 nm wavelength.

Typically, the ArF immersion lithography of 193 nm wavelength uses ahalftone phase shift mask bearing a dot shifter pattern, whereby apattern of holes is formed at the dots after development.

In a preferred embodiment, the exposure step uses halftone phase shiftmasks and includes two exposures of two intersecting sets of lines,whereby a pattern of holes is formed at the intersections between linesafter development.

In a preferred embodiment, the exposure step uses a halftone phase shiftmask bearing a lattice-like shifter pattern, whereby a pattern of holesis formed at the intersections between gratings of the lattice-likeshifter pattern after development.

Most often, the halftone phase shift mask bearing a dot shifter pattern,two intersecting sets of lines, or lattice-like shifter pattern has atransmittance of 3 to 15%.

In a preferred embodiment, the phase shift mask used is a phase shiftmask including a lattice-like first shifter having a line width equal toor less than a half pitch and a second shifter arrayed on the firstshifter and consisting of lines whose on-wafer size is 2 to 30 nmthicker than the line width of the first shifter, whereby a pattern ofholes is formed only where the thick shifter is arrayed.

In another preferred embodiment, the phase shift mask used is a phaseshift mask including a lattice-like first shifter having a line widthequal to or less than a half pitch and a second shifter arrayed on thefirst shifter and consisting of dots whose on-wafer size is 2 to 100 nmthicker than the line width of the first shifter, whereby a pattern ofholes is formed only where the thick shifter is arrayed.

Another embodiment of the pattern forming process comprises the steps ofapplying a resist composition comprising a polymer comprising recurringunits having an acid labile group-substituted hydroxyl group, an acidgenerator, and an organic solvent onto a substrate, heat treating thecomposition to form a resist film, forming a protective film on theresist film, exposing the resist film to high-energy radiation to defineexposed and unexposed regions, heat treating, and applying a developerto the coated substrate to form a negative pattern wherein the unexposedregion of resist film and the protective film are dissolved and theexposed region of resist film is not dissolved.

The protective film is preferably formed of a composition comprising apolymer bearing a 1,1,1,3,3,3-hexafluoro-2-propanol residue and an aminogroup or amine salt-containing compound, or a composition comprising apolymer bearing a 1,1,1,3,3,3-hexafluoro-2-propanol residue and havingamino group or amine salt-containing recurring units copolymerized, thecomposition further comprising an alcohol solvent of at least 4 carbonatoms, an ether solvent of 8 to 12 carbon atoms, or a mixture thereof.

In another aspect, the invention provides a resist compositioncomprising a polymer, an acid generator, and an organic solvent. Thepolymer is dissolvable in a developer selected from the above-describedgroup, adapted to form a negative pattern, and comprises recurring units(a1) and/or (a2) having an acid labile group-substituted hydroxyl grouprepresented by formula (1) defined above, and recurring units derivedfrom a monomer having an adhesive group selected from the groupconsisting of hydroxyl, cyano, carbonyl, ester, ether, lactone,carboxyl, and carboxylic anhydride.

In recurring unit (a1) in formula (1), preferably R² contains anadamantane ring.

In a further aspect, the invention provides an acetal compound havingthe general formula (3):

wherein R¹⁰ is hydrogen or methyl, R¹¹ is a straight, branched orcyclic, tertiary hydrocarbon group of 4 to 20 carbon atoms, R¹² is asingle bond or methylene, and k¹ is an integer of 1 to 3.

ADVANTAGEOUS EFFECTS OF INVENTION

In the process of image formation via positive/negative reversal byorganic solvent development, a photoresist film comprising a polymercomprising recurring units having an acid labile group-substitutedhydroxyl group and an acid generator is characterized by a highdissolution contrast between the unexposed region of promoteddissolution and the exposed region of inhibited dissolution. Bysubjecting this photoresist film to exposure through a mask bearing alattice-like pattern and organic solvent development, a fine holepattern can be formed at a high sensitivity and a high precision ofdimensional control.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 schematically illustrates in cross-sectional views the patternforming process of the invention, FIG. 1A shows a photoresist filmformed on a substrate, FIG. 1B shows the photoresist film being exposed,and FIG. 1C shows the photoresist film being developed with organicsolvent.

FIG. 2 is an optical image of X-direction lines having a pitch of 90 nmand a line size of 45 nm printed under conditions: ArF excimer laser ofwavelength 193 nm, NA 1.3 lens, dipole illumination, 6% halftone phaseshift mask, and s-polarization.

FIG. 3 is an optical image of Y-direction lines like FIG. 2.

FIG. 4 shows a contrast image obtained by overlaying the optical imageof X-direction lines in FIG. 2 with the optical image of Y-directionlines in FIG. 3.

FIG. 5 illustrates a mask bearing a lattice-like pattern.

FIG. 6 is an optical image of a lattice-like pattern having a pitch of90 nm and a line width of 30 nm printed under conditions: NA 1.3 lens,cross-pole illumination, 6% halftone phase shift mask, and azimuthallypolarized illumination.

FIG. 7 illustrates a mask bearing a dot pattern of square dots having apitch of 90 nm and a side width of 60 nm under conditions: NA 1.3 lens,cross-pole illumination, 6% halftone phase shift mask, and azimuthallypolarized illumination.

FIG. 8 is an optical image resulting from the mask of FIG. 7, showingits contrast.

FIG. 9 illustrates a mask bearing a lattice-like pattern having a pitchof 90 nm and a line width of 20 nm on which thick crisscross orintersecting line segments are disposed where dots are to be formed.

FIG. 10 is an optical image resulting from the mask of FIG. 9, showingits contrast.

FIG. 11 illustrates a mask bearing a lattice-like pattern having a pitchof 90 nm and a line width of 15 nm on which thick dots are disposedwhere dots are to be formed.

FIG. 12 is an optical image resulting from the mask of FIG. 11, showingits contrast.

FIG. 13 illustrates a mask without a lattice-like pattern.

FIG. 14 is an optical image resulting from the mask of FIG. 13, showingits contrast.

FIG. 15 is a diagram showing film thickness versus exposure dose inExample 1-1.

FIG. 16 is a diagram showing film thickness versus exposure dose inComparative Example 1-1.

FIG. 17 is a diagram showing film thickness versus exposure dose inComparative Example 1-2.

FIG. 18 illustrates a lattice-like mask used in ArF lithographypatterning test 2.

FIG. 19 illustrates a lattice-like mask with dots disposed atintersections, used in ArF lithography patterning test 3.

FIG. 20 illustrates a mask bearing a lattice-like pattern with thickgratings disposed thereon, used in ArF lithography patterning test 4.

FIG. 21 illustrates an aperture configuration in an exposure tool ofdipole illumination for improving the contrast of X-direction lines.

FIG. 22 illustrates an aperture configuration in an exposure tool ofdipole illumination for improving the contrast of Y-direction lines.

FIG. 23 illustrates an aperture configuration in an exposure tool ofcross-pole illumination for improving the contrast of both X- andY-direction lines.

DESCRIPTION OF EMBODIMENTS

The terms “a” and “an” herein do not denote a limitation of quantity,but rather denote the presence of at least one of the referenced item.

“Optional” or “optionally” means that the subsequently described eventor circumstance may or may not occur, and that the description includesinstances where the event occurs and instances where it does not.

As used herein, the notation (C_(n)-C_(m)) means a group containing fromn to m carbon atoms per group.

The abbreviations and acronyms have the following meaning.

Mw: weight average molecular weightMn: number average molecular weightMw/Mn: molecular weight distribution or dispersityGPC: gel permeation chromatographyPEB: post-exposure bakingPGMEA: propylene glycol monomethyl ether acetate

The abbreviation “phr” refers to parts by weight per 100 parts by weightof resin or polymer.

The invention is directed to a process for forming a pattern viapositive/negative reversal. Specifically the process includes the stepsof applying a photoresist composition based on a polymer comprisingrecurring units having an acid labile group-substituted hydroxyl group,an acid generator, and an organic solvent onto a substrate, prebakingthe composition to remove the unnecessary solvent and form a resistfilm, exposing the resist film to high-energy radiation, baking (PEB),and developing the exposed film with an organic solvent developer toform a negative pattern.

In general, hydroxyl-containing polymers have a lower solubility inorganic solvent than carboxyl-containing polymers. As compared with thepolymers designed to generate carboxyl groups through acid-assistedelimination reaction, the polymers designed to generate hydroxyl groupsthrough acid-assisted elimination reaction have a low solubility inorganic solvent after deprotection, so that a more fraction of film maybe left after patterning. Moreover since these polymers cease todissolve in developer after slight deprotection, they have a highersensitivity than the polymers having an acid labile group-substitutedcarboxyl group. Few study has been made on those polymers designed togenerate hydroxyl groups through deprotection because they are alkaliinsoluble. The inventors have discovered that the hydroxyl group is anoptimum polarity conversion group in organic solvent development.

The recurring units having an acid labile group-substituted hydroxylgroup include recurring units (a1) and (a2) represented by the generalformula (1).

Herein R¹ and R⁴ are each independently hydrogen or methyl. R² is astraight, branched or cyclic, di- to penta-valent aliphatic hydrocarbongroup of 1 to 16 carbon atoms which may contain an ether or esterradical. R³ and R⁵ each are an acid labile group, with the proviso thatwhen R² contains an adamantane ring, an acid labile group of acetal formhaving the general formula (2) is excluded from R³. In formula (2), R⁶is a straight, branched or cyclic, monovalent hydrocarbon group of 1 to10 carbon atoms, R⁷ and R⁸ are each independently hydrogen or astraight, branched or cyclic, monovalent hydrocarbon group of 1 to 10carbon atoms, R⁷ and R⁸ may bond together to form an aliphatichydrocarbon ring with the carbon atom to which they are attached, and R⁹is a straight, branched or cyclic, monovalent hydrocarbon group of 1 to15 carbon atoms. The subscript m is an integer of 1 to 4, a1 and a2 arenumbers in the range: 0≦a1<1.0, 0≦a2<1.0, and 0<a1+a2<1.0.

Examples of suitable monomers from which recurring units (a1) and (a2)are derived are given below wherein R¹, R³, R⁴, and R⁵ are as definedabove.

Of these monomers, those monomers having an adamantane ring as R² informula (1) are preferred for etch resistance.

Onishi parameter is used as an index of etch resistance. For a monomercorresponding to the recurring unit, Onishi parameter is computed usingthe number of all atoms as a numerator and the number of carbon atomsminus the number of oxygen atoms as a denominator. A value of Onishiparameter as computed after deprotection is 6.0 for methacrylic acid,3.4 for hydroxyadamantane methacrylate, and 4.8 for dihydroxyadamantanemethacrylate. Smaller values of Onishi parameter indicate higher etchresistance. The films using inventive base polymers have higher etchresistance than the negative development film using a prior art basepolymer capable of generating methacrylic acid through deprotection.

Of the monomers having an adamantane ring as R², acetal compounds havingthe general formula (3) are preferred.

Herein R¹⁰ is hydrogen or methyl, R¹¹ is a straight, branched or cyclic,tertiary hydrocarbon group of 4 to 20 carbon atoms, R¹² is a single bondor methylene, and k¹ is an integer of 1 to 3.

The above monomer is hydroxyadamantane methacrylate whose hydroxyl groupis substituted by methylene acetal. By copolymerizing the monomer, thereare obtained advantages such as excellent dissolution contrast duringorganic solvent development and control of acid diffusion. By virtue ofimproved dissolution contrast and controlled acid diffusion, a holepattern after development is improved in dimensional uniformity.

In formula (3), R¹¹ is a straight, branched or cyclic, tertiaryhydrocarbon group of 4 to 20 carbon atoms, examples of which include,but are not limited to, tert-butyl, tert-amyl, 1,1-diethylpropyl,2-cyclopentylpropan-2-yl, 2-cyclohexylpropan-2-yl,2-(bicyclo[2.2.1]heptan-2-yl)propan-2-yl, 2-(adamantan-1-yl)propan-2-yl,2-(tricyclo[5.2.1.0^(2,6)]decan-8-yl)propan-2-yl,2-(tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodecan-3-yl)propan-2-yl,1-ethylcyclopentyl, 1-butylcyclopentyl, 1-ethylcyclohexyl,1-butylcyclohexyl, 1-ethyl-2-cyclopentenyl, 1-ethyl-2-cyclohexenyl,2-methyl-2-adamantyl, 2-ethyl-2-adamantyl,8-methyl-8-tricyclo[5.2.1.0^(2,6)]decyl,8-ethyl-8-tricyclo[5.2.1.0^(2,6)]decyl,3-methyl-3-tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodecyl, and3-ethyl-3-tetracyclo[4.4.0.1^(2,5).1^(7,10)]dodecyl.

The subscript k¹ is an integer of 1 to 3, preferably 1 or 2. Thoseacetal compounds wherein k¹ is 3 themselves become high molecular weightones, which may sometimes be difficult to purify by distillation.

Examples of the acetal compound having formula (3) are shown below, butnot limited thereto. Herein, R¹⁰ is as defined above.

The acetal compound having formula (3) may be obtained, for example,through steps (i) and (ii) according to the following scheme althoughthe synthesis route is not limited thereto.

Herein R¹⁰ to R¹², and k¹ are as defined above, and T³ is halogen.

Step (i) is a reaction of an alcohol compound (4) with HCHO orequivalent and hydrogen halide, to form a haloalkyl ether compound (5).Typical of halogen T³ are chlorine, bromine and iodine. Of these,chlorine is most preferred for ease of handling. The reaction may beconducted in a solventless system or in a solvent. Depending on reactionconditions, a suitable solvent may be selected from among ethers such astetrahydrofuran, diethyl ether, di-n-butyl ether, and 1,4-dioxane,hydrocarbons such as n-hexane, n-heptane, benzene, toluene, xylene, andcumene, aprotic polar solvents such as dimethyl sulfoxide (DMSO) andN,N-dimethylformamide (DMF), and chlorinated organic solvents such asmethylene chloride, chloroform, and carbon tetrachloride, alone or inadmixture.

Reaction conditions including temperature and time may widely vary. Inan example where T³ is chlorine, the reaction temperature is desirablyset in a range of −30° C. to 80° C., more desirably −20° C. to 20° C.for rapid reaction to completion. The reaction time is determined asappropriate by monitoring the reaction process by gas chromatography(GC) or silica gel thin-layer chromatography (TLC) because it isdesirable from the yield aspect to drive the reaction to completion.Usually the reaction time is about 0.1 to about 10 hours. If necessary,the compound may be purified from the reaction mixture by standardtechniques like distillation, chromatography and recrystallization. Mostoften, the crude product has a sufficient purity as the reactant to thesubsequent step and may be used in the subsequent step withoutpurification.

Step (ii) is a protection reaction of haloalkyl ether compound (5) witha starting alcohol compound (6) to form an acetal compound (3). Thereaction may be conducted in a standard way, for example, in asolventless system or in a solvent, by adding alcohol compound (6),haloalkyl ether compound (5), and a base such as triethylamine, pyridineor 4-dimethylaminopyridine in sequence or at the same time, and optionalcooling or heating.

The amount of haloalkyl ether compound (5) used is preferably 0.5 to 10moles, more preferably 1.0 to 2.0 moles per mole of alcohol compound(6), when the starting alcohol compound (6) has k¹=1. If the amount ofcompound (5) is less than 0.5 mole, a large fraction of the startingcompound is left unreacted, leading to a substantial drop of yield. Morethan 10 moles of compound (5) may be disadvantageous from the costaspect because of an increased reactant expense and a reduced pot yield.

Examples of the solvent used herein include hydrocarbons such astoluene, xylene, hexane, and heptane, chlorinated solvents such asmethylene chloride, chloroform, and dichloroethane, ethers such asdiethyl ether, tetrahydrofuran, and dibutyl ether, ketones such asacetone and 2-butanone, esters such as ethyl acetate and butyl acetate,nitriles such as acetonitrile, alcohols such as methanol and ethanol,aprotic polar solvents such as N,N-dimethylformamide,N,N-dimethylacetamide and dimethyl sulfoxide, and water, alone or inadmixture. A phase transfer catalyst such as tetrabutylammonium hydrogensulfate may be added to the reaction system. The amount of phasetransfer catalyst used may be preferably 0.0001 to 1.0 mole, and morepreferably 0.001 to 0.5 mole per mole of alcohol compound (6) wherein k¹is 1. Less than 0.0001 mole of the catalyst may fail to achieve thedesired addition effect whereas more than 1.0 mole may be uneconomicaldue to an increased material expense.

It is desired for higher yields that the reaction time be determined bymonitoring the progress of reaction by thin-layer chromatography (TLC)or gas chromatography (GC). The reaction time is usually about 30minutes to about 40 hours. The acetal compound (3) may be recovered fromthe reaction mixture by ordinary aqueous work-up. If necessary, it canbe purified by any standard technique such as distillation,recrystallization or chromatography.

As the base resin used in the resist composition suited for organicsolvent development for achieving positive/negative reversal in thepatterning process, a polymer or high molecular weight compoundcomprising recurring units (a1) or (a2) having an acid labilegroup-substituted hydroxyl group represented by formula (1) ispreferably used.

The polymer comprising recurring units (a1) and/or (a2) may havecopolymerized therewith recurring units (b) having an acid labilegroup-substituted carboxyl group as represented by the followingformula.

Suitable monomers Mb from which recurring units (b) are derived have thefollowing formula.

Herein R¹³ is hydrogen or methyl, R¹⁴ is an acid labile group, and Z isa single bond or —C(═O)—O—R¹⁵— wherein R¹⁵ is a straight, branched orcyclic C₁-C₁₀ alkylene group which may contain an ether, ester, lactoneor hydroxyl radical, or a naphthylene group.

Examples of the monomers Mb having different Z structures are givenbelow wherein R¹³ and R¹⁴ are as defined above.

The acid labile groups represented by R³ and R⁵ in formula (1) and R¹⁴in recurring unit (b) or Monomer Mb may be the same or different andselected from a variety of such groups. Suitable acid labile groupsinclude groups of the following formula (AL-10), acetal groups of thefollowing formula (AL-11), tertiary alkyl groups of the followingformula (AL-12), and oxoalkyl groups of 4 to 20 carbon atoms, but arenot limited thereto.

In formulae (AL-10) and (AL-11), R⁵¹ and R⁵⁴ each are a monovalenthydrocarbon group, typically straight, branched or cyclic alkyl group,of 1 to 40 carbon atoms, more specifically 1 to 20 carbon atoms, whichmay contain a heteroatom such as oxygen, sulfur, nitrogen or fluorine.R⁵² and R⁵³ each are hydrogen or a monovalent hydrocarbon group,typically straight, branched or cyclic alkyl group, of 1 to 20 carbonatoms which may contain a heteroatom such as oxygen, sulfur, nitrogen orfluorine. The subscript “a5” is an integer of 0 to 10, and especially 1to 5. Alternatively, a pair of R⁵² and R⁵³, R⁵² and R⁵⁴, or R⁵³ and R⁵⁴may bond together to form a ring, specifically aliphatic ring, with thecarbon atom or the carbon and oxygen atoms to which they are attached,the ring having 3 to 20 carbon atoms, especially 4 to 16 carbon atoms.If formula (AL-10) wherein a5 is at least 1 is applied to the acidlabile groups R³ and R⁵, a carboxyl group is generated followingdeprotection. Thus formula (AL-10) representing R³ and R⁵ is limited toa5=0, whereas formula (AL-10) representing R¹⁴ need not be limited toa5=0.

In formula (AL-12), R⁵⁵, R⁵⁶ and R⁵⁷ each are a monovalent hydrocarbongroup, typically straight, branched or cyclic alkyl group, of 1 to 20carbon atoms which may contain a heteroatom such as oxygen, sulfur,nitrogen or fluorine. Alternatively, a pair of R⁵⁵ and R⁵⁶, R⁵⁵ and R⁵⁷,or R⁵⁶ and R⁵⁷ may bond together to form a ring, specifically aliphaticring, with the carbon atom to which they are attached, the ring having 3to 20 carbon atoms, especially 4 to 16 carbon atoms.

Illustrative examples of the groups of formula (AL-10) includetert-butoxycarbonyl, tert-butoxycarbonylmethyl, tert-amyloxycarbonyl,tert-amyloxycarbonylmethyl, 1-ethoxyethoxycarbonylmethyl,2-tetrahydropyranyloxycarbonylmethyl and2-tetrahydrofuranyloxycarbonylmethyl as well as substituent groups ofthe following formulae (AL-10)-1 to (AL-10)-10.

In formulae (AL-10)-1 to (AL-10)-10, R⁵⁸ is each independently astraight, branched or cyclic C₁-C₈ alkyl group, C₆-C₂₀ aryl group orC₇-C₂₀ aralkyl group; R⁵⁹ is hydrogen or a straight, branched or cyclicC₁-C₂₀ alkyl group; R^(H) is a C₆-C₂₀ aryl group or C₇-C₂₀ aralkylgroup; and “a5” is as defined above.

Illustrative examples of the acetal group of formula (AL-11) includethose of the following formulae (AL-11)-1 to (AL-11)-44.

As noted above, groups of formulae (AL-11)-15 and (AL-11)-35 to 44 areexcluded from R³ when R² in formula (1) contains an adamantane ring.

Other examples of acid labile groups include those of the followingformula (AL-11a) or (AL-11b) while the polymer may be crosslinked withinthe molecule or between molecules with these acid labile groups.

Herein R⁶¹ and R⁶² each are hydrogen or a straight, branched or cyclicC₁-C₈ alkyl group, or R⁶¹ and R⁶² may bond together to form a ring withthe carbon atom to which they are attached, and R⁶¹ and R⁶² are straightor branched C₁-C₈ alkylene groups when they form a ring. R⁶³ is astraight, branched or cyclic C₁-C₁₀ alkylene group. Each of b5 and d5 is0 or an integer of 1 to 10, preferably 0 or an integer of 1 to 5, and c5is an integer of 1 to 7. “A” is a (c5+1)-valent aliphatic or alicyclicsaturated hydrocarbon group, aromatic hydrocarbon group or heterocyclicgroup having 1 to 50 carbon atoms, which may be separated by aheteroatom such as oxygen, sulfur or nitrogen or in which some of thehydrogen atoms attached to carbon atoms may be substituted by hydroxyl,carboxyl, carbonyl radicals or fluorine atoms. “B” is —CO—O—, —NHCO—O—or —NHCONH—.

Preferably, “A” is selected from divalent to tetravalent, straight,branched or cyclic C₁-C₂₀ alkylene, alkanetriyl and alkanetetraylgroups, and C₆-C₃₀ arylene groups, which may be separated by aheteroatom such as oxygen, sulfur or nitrogen or in which some of thehydrogen atoms attached to carbon atoms may be substituted by hydroxyl,carboxyl, acyl radicals or halogen atoms. The subscript c5 is preferablyan integer of 1 to 3.

The crosslinking acetal groups of formulae (AL-11a) and (AL-11b) areexemplified by the following formulae (AL-11)-60 through (AL-11)-67.

Illustrative examples of the tertiary alkyl of formula (AL-12) includetert-butyl, triethylcarbyl, 1-ethylnorbornyl, 1-methylcyclohexyl,1-ethylcyclopentyl, and tert-amyl groups as well as those of (AL-12)-1to (AL-12)-16.

Herein R⁶⁴ is each independently a straight, branched or cyclic C₁-C₈alkyl group, C₆-C₂₀ aryl group or C₇-C₂₀ aralkyl group, two R^(H) maybond together to form a ring; R⁶⁵ and R⁶⁷ each are hydrogen or astraight, branched or cyclic C₁-C₂₀ alkyl group; and R⁶⁶ is a C₆-C₂₀aryl group or C₇-C₂₀ aralkyl group.

With acid labile groups containing R⁶⁸ representative of a di- orpoly-valent alkylene or arylene group as shown by formulae (AL-12)-17and (AL-12)-18, the polymer may be crosslinked within the molecule orbetween molecules. In formulae (AL-12)-17 and (AL-12)-18, R⁶⁴ is asdefined above, R⁶⁸ is a single bond, or a straight, branched or cyclicC₁-C₂₀ alkylene group or arylene group which may contain a heteroatomsuch as oxygen, sulfur or nitrogen, and b6 is an integer of 0 to 3.Formula (AL-12)-17 is applicable to all acid labile groups R³, R⁵, andR¹⁴ while formula (AL-12)-18 is applicable to only R¹⁴.

The groups represented by R⁶⁴, R⁶⁵, R⁶⁶ and R⁶⁷ may contain a heteroatomsuch as oxygen, nitrogen or sulfur. Such groups are exemplified by thoseof the following formulae (AL-13)-1 to (AL-13)-7.

As the acid labile group R¹⁴, groups having an exo-form structurerepresented by the formula (AL-12)-19 are also preferred.

Herein, R⁶⁹ is a straight, branched or cyclic C₁-C₈ alkyl group or anoptionally substituted C₆-C₂₀ aryl group; R⁷⁰ to R⁷⁵, R⁷⁸ and R⁷⁹ areeach independently hydrogen or a monovalent hydrocarbon group, typicallyalkyl, of 1 to 15 carbon atoms which may contain a heteroatom; and R⁷⁶and R⁷⁷ are hydrogen or a monovalent hydrocarbon group of 1 to 15 carbonatoms which may contain a heteroatom. Alternatively, a pair of R⁷⁰ andR⁷¹, R⁷² and R⁷⁴, R⁷² and R⁷⁵, R⁷³ and R⁷⁵, R⁷³ and R⁷⁹, R⁷⁴ and R⁷⁸,R⁷⁶ and R⁷⁷, or R⁷⁷ and R⁷⁸ may bond together to form a ring,specifically aliphatic ring, with the carbon atom(s) to which they areattached, and in this case, each ring-former R is a divalent hydrocarbongroup, typically alkylene, of 1 to 15 carbon atoms which may contain aheteroatom. Also, a pair of R⁷⁰ and R⁷⁹, R⁷⁶ and R⁷⁹, or R⁷² and R⁷⁴which are attached to vicinal carbon atoms may bond together directly toform a double bond. The formula also represents an enantiomer.

The ester form monomers from which recurring units having an exo-formstructure represented by the formula (AL-12)-19 are derived aredescribed in U.S. Pat. No. 6,448,420 (JP-A 2000-327633), with suchrecurring units being illustrated below.

It is noted that R¹¹¹ and R¹¹² are each independently hydrogen, methyl,—COOCH₃, —CH₂COOCH₃ or the like. Illustrative non-limiting examples ofsuitable monomers are given below.

Also included in the acid labile groups of formula (AL-12) as R¹⁴ areacid labile groups having furandiyl, tetrahydrofurandiyl oroxanorbornanediyl as represented by the following formula (AL-12)-20.

Herein, R⁸⁰ and R⁸¹ are each independently a monovalent hydrocarbongroup, typically a straight, branched or cyclic C₁-C₁₀ alkyl. R^(H) andR⁸¹ may bond together to form an aliphatic hydrocarbon ring of 3 to 20carbon atoms with the carbon atom to which they are attached. R⁸² is adivalent group selected from furandiyl, tetrahydrofurandiyl andoxanorbornanediyl. R⁸³ is hydrogen or a monovalent hydrocarbon group,typically a straight, branched or cyclic C₁-C₁₀ alkyl, which may containa heteroatom.

Examples of the monomers from which the recurring units substituted withacid labile groups having furandiyl, tetrahydrofurandiyl andoxanorbornanediyl as represented by the formula:

(wherein R⁸⁰ to R⁸³, and R¹¹² are as defined above) are derived areshown below. Note that Me is methyl and Ac is acetyl.

While the polymer used herein preferably includes recurring units (a1)and (a2) in formula (1) and optional recurring units (b), it may havefurther copolymerized therein recurring units (c) derived from monomershaving a adhesive group such as hydroxy, cyano, carbonyl, ester, ether,lactone, carboxyl, carboxylic anhydride, sulfonic ester, and disulfonegroup. Inter alia, monomers having a lactone ring as the adhesive groupare most preferred.

Examples of monomers from which recurring units (c) are derived aregiven below.

In a preferred embodiment, the copolymer has further copolymerizedtherein units selected from sulfonium salts (d1) to (d3) represented bythe general formulae below.

Herein R²⁰, R²⁴, and R^(H) each are hydrogen or methyl, R²¹ is a singlebond, phenylene, —O—R³³—, or —C(═O)—Y—R³³—, Y is oxygen or NH, R³³ is astraight, branched or cyclic C₁-C₆ alkylene group, alkenylene orphenylene group, which may contain a carbonyl (—CO—), ester (—COO—),ether (—O—) or hydroxyl radical, R²², R²³, R²⁵, R²⁶, R²⁷, R²⁹, R³⁰, andR³¹ each independently a straight, branched or cyclic C₁-C₁₂ alkyl groupwhich may contain a carbonyl, ester or ether radical, or a C₆-C₁₂ aryl,C₇-C₂₀ aralkyl, or thiophenyl group, Z₀ is a single bond, methylene,ethylene, phenylene, fluorophenylene, —O—R³²—, or —C(═O)—Z₁—R^(H)—, Z₁is oxygen or NH, R³² is a straight, branched or cyclic C₁-C₆ alkylenegroup, alkenylene or phenylene group, which may contain a carbonyl,ester, ether or hydroxyl radical, M⁻ is a non-nucleophilic counter ion,d1, d2 and d3 are in the range of 0≦d1≦0.3, 0≦d2≦0.3, 0≦d3≦0.3, and0≦d1+d2+d3≦0.3.

In the copolymer, the recurring units (a1), (a2), (b), (c), (d1), (d2),and (d3) are preferably present in proportions: 0≦a1<1.0, 0≦a2<1.0,0≦b<1.0, 0<a1+a2<1.0, 0≦c<1.0, 0≦d1<0.2, 0≦d2<0.2, and 0≦d3<0.2, andmore preferably 0≦a1≦0.8, 0≦a2≦0.8, 0≦b≦0.75, 0.1≦a1+a2 0.8, 0≦c≦0.8,0≦d1<0.15, 0≦d2<0.15, and 0≦d3<0.15, provided that a1+a2+b+c+d1+d2+d3=1.

It is noted that the meaning of a+b=1, for example, is that in a polymercomprising recurring units (a) and (b), the sum of recurring units (a)and (b) is 100 mol % based on the total amount of entire recurringunits. The meaning of a+b<1 is that the sum of recurring units (a) and(b) is less than 100 mol % based on the total amount of entire recurringunits, indicating the inclusion of other recurring units, for example,units (c).

The polymer serving as the base resin in the resist composition used inthe pattern forming process of the invention should preferably have aweight average molecular weight (Mw) in the range of 1,000 to 500,000,and more preferably 2,000 to 30,000, as measured in tetrahydrofuransolvent by GPC using polystyrene standards. With too low a Mw, filmslimming is likely to occur upon organic solvent development. A polymerwith too high a Mw may lose solubility in organic solvent and have alikelihood of footing after pattern formation.

If a polymer has a broad molecular weight distribution or dispersity(Mw/Mn), which indicates the presence of lower and higher molecularweight polymer fractions, there is a possibility that followingexposure, foreign matter is left on the pattern or the pattern profileis exacerbated. The influences of molecular weight and dispersity becomestronger as the pattern rule becomes finer. Therefore, themulti-component copolymer should preferably have a narrow dispersity(Mw/Mn) of 1.0 to 2.0, especially 1.0 to 1.5, in order to provide aresist composition suitable for micropatterning to a small feature size.

It is acceptable to use a blend of two or more polymers which differ incompositional ratio, molecular weight or dispersity or a blend of apolymer having an acid labile group-substituted hydroxyl group andanother polymer free of an acid labile group-substituted hydroxyl group.

The polymer as used herein may be synthesized by any desired method, forexample, by dissolving unsaturated bond-containing monomerscorresponding to the respective units (a1), (a2), (b), (c), (d1), (d2),and (d3) in an organic solvent, adding a radical initiator thereto, andeffecting heat polymerization. Examples of the organic solvent which canbe used for polymerization include toluene, benzene, tetrahydrofuran,diethyl ether and dioxane. Examples of the polymerization initiator usedherein include 2,2′-azobisisobutyronitrile (AIBN),2,2′-azobis(2,4-dimethylvaleronitrile), dimethyl2,2-azobis(2-methylpropionate), benzoyl peroxide, and lauroyl peroxide.Preferably the system is heated at 50 to 80° C. for polymerization totake place. The reaction time is 2 to 100 hours, preferably 5 to 20hours. The acid labile group that has been incorporated in the monomermay be kept as such, or the acid labile group may be once eliminatedwith an acid catalyst and thereafter protected or partially protected.

As described previously, the pattern forming process of the inventioncomprises the steps of coating the positive resist composition describedabove onto a substrate, prebaking the resist composition to form aresist film, exposing a selected region of the resist film tohigh-energy radiation, baking (PEB), and developing the exposed resistfilm with an organic solvent developer so that the unexposed region offilm is dissolved and the exposed region of film is left, therebyforming a negative tone resist pattern such as a hole or trench pattern.

For the purpose of forming a negative tone pattern from a polymer,recurring units (a1) and/or (a2) having an acid labile group-substitutedhydroxyl group are essential, whereas recurring units (b) having an acidlabile group-substituted carboxyl group are not necessarily needed.Specifically, the recurring units (b) having an acid labilegroup-substituted carboxyl group are preferably incorporated in anamount of 0 mol % to 90 mol %, more preferably 5 mol % to 80 mol % ofthe total recurring units.

The resist composition used in the pattern forming process of theinvention may further comprise an organic solvent, a compound capable ofgenerating an acid in response to high-energy radiation (known as “acidgenerator”), and optionally, a dissolution regulator, basic compound,surfactant, acetylene alcohol, and other components.

The resist composition used herein may include an acid generator inorder for the composition to function as a chemically amplified positiveresist composition. Typical of the acid generator used herein is aphotoacid generator (PAG) capable of generating an acid in response toactinic light or radiation. The PAG may preferably be compounded in anamount of 0.5 to 30 parts and more preferably 1 to 20 parts by weightper 100 parts by weight of the base resin. The PAG is any compoundcapable of generating an acid upon exposure to high-energy radiation.Suitable PAGs include sulfonium salts, iodonium salts,sulfonyldiazomethane, N-sulfonyloxyimide, and oxime-O-sulfonate acidgenerators. The PAGs may be used alone or in admixture of two or more.Exemplary acid generators are described in U.S. Pat. No. 7,537,880 (JP-A2008-111103, paragraphs [0122] to [0142]). Where the polymer has apolymerizable acid generator unit selected from recurring units (d1),(d2) and (d3) copolymerized therein, the acid generator need notnecessarily be added.

Examples of the organic solvent used herein are described in JP-A2008-111103, paragraphs [0144] to [0145] (U.S. Pat. No. 7,537,880).Specifically, exemplary solvents include ketones such as cyclohexanoneand methyl-2-n-amyl ketone; alcohols such as 3-methoxybutanol,3-methyl-3-methoxybutanol, 1-methoxy-2-propanol, and1-ethoxy-2-propanol; ethers such as propylene glycol monomethyl ether,ethylene glycol monomethyl ether, propylene glycol monoethyl ether,ethylene glycol monoethyl ether, propylene glycol dimethyl ether, anddiethylene glycol dimethyl ether; esters such as propylene glycolmonomethyl ether acetate (PGMEA), propylene glycol monoethyl etheracetate, ethyl lactate, ethyl pyruvate, butyl acetate, methyl3-methoxypropionate, ethyl 3-ethoxypropionate, tert-butyl acetate,tert-butyl propionate, and propylene glycol mono-tert-butyl etheracetate; and lactones such as γ-butyrolactone, and mixtures thereof.Where an acid labile group of acetal form is used, a high-boilingalcohol solvent such as diethylene glycol, propylene glycol, glycerol,1,4-butane diol or 1,3-butane diol may be added for acceleratingdeprotection reaction of acetal. Exemplary basic compounds includeprimary, secondary and tertiary amine compounds, specifically aminecompounds having a hydroxyl, ether, ester, lactone, cyano or sulfonategroup, as described in JP-A 2008-111103, paragraphs [0146] to [0164],and compounds having a carbamate group, as described in JP 3790649.

Onium salts such as sulfonium salts, iodonium salts and ammonium saltsof sulfonic acids which are not fluorinated at α-position as describedin US 2008153030 (JP-A 2008-158339) and similar onium salts ofcarboxylic acid may be used as the quencher. While an α-positionfluorinated sulfonic acid, imide acid, and methide acid are necessary todeprotect the acid labile group of carboxylic acid ester, an α-positionnon-fluorinated sulfonic acid and a carboxylic acid are released by saltexchange with an onium salt which is not fluorinated at α-position. Anα-position non-fluorinated sulfonic acid and a carboxylic acid functionas a quencher because they do not induce deprotection reaction. Inparticular, since sulfonium salts and iodonium salts of an α-positionnon-fluorinated sulfonic acid and a carboxylic acid arephoto-decomposable, those portions receiving a high light intensity arereduced in quenching capability and increased in the concentration of aα-position fluorinated sulfonic acid, imide acid, or methide acid. As aresult, the exposed portions are improved in contrast. When a negativetone pattern is formed using an organic solvent, the improvement in thecontrast of exposed portions leads to an improvement in therectangularity of negative pattern. Onium salts including sulfoniumsalts, iodonium salts and ammonium salts of an α-positionnon-fluorinated sulfonic acid and a carboxylic acid are highly effectivein controlling the diffusion of an α-position fluorinated sulfonic acid,imide acid and methide acid. This is because the onium salt resultingfrom salt exchange is less mobile due to a higher molecular weight. Inthe event that a hole pattern is formed by negative development, sinceacid is generated in many regions, it is very important to control thediffusion of acid from the exposed area to the unexposed area. Theaddition of onium salts including sulfonium salts, iodonium salts andammonium salts of an α-position non-fluorinated sulfonic acid and acarboxylic acid as well as the carbamate compound capable of generatingan amine compound under the action of acid is very important from theaspect of controlling acid diffusion.

Exemplary surfactants are described in JP-A 2008-111103, paragraphs[0165] to [0166]. Exemplary dissolution regulators are described in JP-A2008-122932 (US 2008090172), paragraphs [0155] to [0178], and exemplaryacetylene alcohols in paragraphs [0179] to [0182].

Also a polymeric additive may be added for improving the waterrepellency on surface of a resist film as spin coated. This additive maybe used in the topcoatless immersion lithography. These additives have aspecific structure with a 1,1,1,3,3,3-hexafluoro-2-propanol residue andare described in JP-A 2007-297590 and JP-A 2008-111103. The waterrepellency improver to be added to the resist should be soluble in theorganic solvent as the developer. The water repellency improver ofspecific structure with a 1,1,1,3,3,3-hexafluoro-2-propanol residue iswell soluble in the developer. A polymer having an amino group or aminesalt copolymerized as recurring units may serve as the water repellentadditive and is effective for preventing evaporation of acid during PEBand avoiding any hole pattern opening failure after development. Anappropriate amount of the water repellency improver is 0.1 to 20 parts,preferably 0.5 to 10 parts by weight per 100 parts by weight of the baseresin.

Notably, an appropriate amount of the organic solvent is 100 to 10,000parts, preferably 300 to 8,000 parts by weight, and an appropriateamount of the basic compound is 0.0001 to 30 parts, preferably 0.001 to20 parts by weight, per 100 parts by weight of the base resin.

Process

Now referring to the drawings, the pattern forming process of theinvention is illustrated in FIG. 1. First, the positive resistcomposition is coated on a substrate to form a resist film thereon.Specifically, a resist film 40 of a positive resist composition isformed on a processable substrate 20 disposed on a substrate 10 directlyor via an intermediate intervening layer 30 as shown in FIG. 1A. Theresist film preferably has a thickness of 10 to 1,000 nm and morepreferably 20 to 500 nm. Prior to exposure, the resist film is heated orprebaked, preferably at a temperature of 60 to 180° C., especially 70 to150° C. for a time of 10 to 300 seconds, especially 15 to 200 seconds.

The substrate 10 used herein is generally a silicon substrate. Theprocessable substrate (or target film) 20 used herein includes SiO₂,SiN, SiON, SiOC, p-Si, α-Si, TiN, WSi, BPSG, SOG, Cr, CrO, CrON, MoSi,low dielectric film, and etch stopper film. The intermediate interveninglayer 30 includes hard masks of SiO₂, SiN, SiON or p-Si, an undercoat inthe form of carbon film, a silicon-containing intermediate film, and anorganic antireflective coating.

Next comes exposure depicted at 50 in FIG. 1B. For the exposure,preference is given to high-energy radiation having a wavelength of 140to 250 nm and EUV having a wavelength of 13.5 nm, and especially ArFexcimer laser radiation of 193 nm. The exposure may be done either in adry atmosphere such as air or nitrogen stream or by immersionlithography in water. The ArF immersion lithography uses deionized wateror liquids having a refractive index of at least 1 and highlytransparent to the exposure wavelength such as alkanes as the immersionsolvent. The immersion lithography involves prebaking a resist film andexposing the resist film to light through a projection lens, with waterintroduced between the resist film and the projection lens. Since thisallows lenses to be designed to a NA of 1.0 or higher, formation offiner feature size patterns is possible. The immersion lithography isimportant for the ArF lithography to survive to the 45-nm node. In thecase of immersion lithography, deionized water rinsing (or post-soaking)may be carried out after exposure for removing water droplets left onthe resist film, or a protective film may be applied onto the resistfilm after pre-baking for preventing any leach-out from the resist filmand improving water slip on the film surface.

The resist protective film used in the immersion lithography ispreferably formed from a solution of a polymer having1,1,1,3,3,3-hexafluoro-2-propanol residues which is insoluble in water,but soluble in an alkaline developer liquid, in a solvent selected fromalcohols of at least 4 carbon atoms, ethers of 8 to 12 carbon atoms, andmixtures thereof. The protective film-forming composition used hereinmay be based on a polymer comprising recurring units derived from amonomer having a 1,1,1,3,3,3-hexafluoro-2-propanol residue. While theprotective film must dissolve in the organic solvent developer, thepolymer comprising recurring units derived from a monomer having a1,1,1,3,3,3-hexafluoro-2-propanol residue dissolves in organic solventdevelopers. In particular, protective film-forming materials having1,1,1,3,3,3-hexafluoro-2-propanol residues as described in JP-A2007-025634 and JP-A 2008-003569 readily dissolve in organic solventdevelopers.

In the protective film-forming composition, an amine compound or aminesalt or a polymer having copolymerized therein recurring unitscontaining an amine compound or amine salt may be used. This componentis effective for controlling diffusion of the acid generated in theexposed region of the photoresist film to the unexposed region forthereby preventing any hole opening failure. Useful protective filmmaterials having an amine compound added thereto are described in JP-A2008-003569, and useful protective film materials having an amino groupor amine salt copolymerized are described in JP-A 2007-316448. The aminecompound or amine salt may be selected from the compounds enumerated asthe basic compound to be added to the resist composition. An appropriateamount of the amine compound or amine salt added is 0.01 to 10 parts,preferably 0.02 to 8 parts by weight per 100 parts by weight of the baseresin.

After formation of the photoresist film, deionized water rinsing (orpost-soaking) may be carried out for extracting the acid generator andthe like from the film surface or washing away particles, or afterexposure, rinsing (or post-soaking) may be carried out for removingwater droplets left on the resist film. If the acid evaporating from theexposed region during PEB deposits on the unexposed region to deprotectthe protective group on the surface of the unexposed region, there is apossibility that the surface edges of holes after development arebridged to close the holes. Particularly in the case of negativedevelopment, regions surrounding the holes receive light so that acid isgenerated therein. There is a possibility that the holes are not openedif the acid outside the holes evaporates and deposits inside the holesduring PEB. Provision of a protective film is effective for preventingevaporation of acid and for avoiding any hole opening failure. Aprotective film having an amine compound added thereto is more effectivefor preventing acid evaporation. On the other hand, a protective film towhich an acid compound such as a carboxyl or sulfo group is added orwhich is based on a polymer having copolymerized therein monomeric unitscontaining a carboxyl or sulfo group is undesirable because of apotential hole opening failure.

The other embodiment of the invention is a process for forming a patternby applying a resist composition comprising a polymer comprisingrecurring units having an acid labile group-substituted hydroxyl group,an acid generator, and an organic solvent onto a substrate, baking thecomposition to form a resist film, forming a protective film on theresist film, exposing the resist film to high-energy radiation to defineexposed and unexposed regions, baking, and applying a developer to thecoated substrate to form a negative pattern wherein the unexposed regionof resist film and the protective film are dissolved and the exposedregion of resist film is not dissolved. The protective film ispreferably formed from a composition comprising a polymer bearing a1,1,1,3,3,3-hexafluoro-2-propanol residue and an amino group or aminesalt-containing compound, or a composition comprising a polymer bearinga 1,1,1,3,3,3-hexafluoro-2-propanol residue and having amino group oramine salt-containing recurring units copolymerized, the compositionfurther comprising an alcohol solvent of at least 4 carbon atoms, anether solvent of 8 to 12 carbon atoms, or a mixture thereof.

With respect to the recurring units having a1,1,1,3,3,3-hexafluoro-2-propanol residue, those monomers having a—C(CF₃)(OH) group, i.e., a carbon atom having CF₃ and OH radicals bondedthereto (some monomers in the last but one and two lists, and allmonomers in the last list) are preferably selected among the exemplarymonomers listed for the recurring unit (c). The amino group-containingcompound may be selected from the exemplary amine compounds (to be addedto photoresist compositions) described in JP-A 2008-111103, paragraphs[0146] to [0164]. As the amine salt-containing compound, salts of theforegoing amine compounds with carboxylic acid or sulfonic acid may beused.

Suitable alcohols of at least 4 carbon atoms include 1-butyl alcohol,2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol,2-pentanol, 3-pentanol, tert-amyl alcohol, neopentyl alcohol,2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol,cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol,3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol,2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol,3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol,cyclohexanol, and 1-octanol. Suitable ether solvents of 8 to 12 carbonatoms include di-n-butyl ether, diisobutyl ether, di-sec-butyl ether,di-n-pentyl ether, diisopentyl ether, di-sec-pentyl ether, di-t-amylether, and di-n-hexyl ether.

Exposure is preferably performed in an exposure dose of about 1 to 200mJ/cm², more preferably about 10 to 100 mJ/cm². This is followed bybaking (PEB) on a hot plate at 60 to 150° C. for 1 to 5 minutes,preferably at 80 to 120° C. for 1 to 3 minutes.

Thereafter the exposed resist film is developed with a developerconsisting of an organic solvent for 0.1 to 3 minutes, preferably 0.5 to2 minutes by any conventional techniques such as dip, puddle and spraytechniques. In this way, the unexposed region of resist film wasdissolved away, leaving a negative resist pattern 40 on the substrate 10as shown in FIG. 1C. The developer used herein is preferably selectedfrom among ketones such as 2-octanone, 2-nonanone, 2-heptanone,3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone,methylcyclohexanone, acetophenone, and methylacetophenone, and esterssuch as propyl acetate, butyl acetate, isobutyl acetate, amyl acetate,butenyl acetate, isoamyl acetate, phenyl acetate, propyl formate, butylformate, isobutyl formate, amyl formate, isoamyl formate, methylvalerate, methyl pentenoate, methyl crotonate, ethyl crotonate, methyllactate, ethyl lactate, propyl lactate, butyl lactate, isobutyl lactate,amyl lactate, isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate,benzyl acetate, methyl phenylacetate, benzyl formate, phenylethylformate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate, and mixtures thereof.

At the end of development, the resist film is rinsed. As the rinsingliquid, a solvent which is miscible with the developer and does notdissolve the resist film is preferred. Suitable solvents includealcohols of 3 to 10 carbon atoms, ether compounds of 8 to 12 carbonatoms, alkanes, alkenes, and alkynes of 6 to 12 carbon atoms, andaromatic solvents. Specifically, suitable alkanes of 6 to 12 carbonatoms include hexane, heptane, octane, nonane, decane, undecane,dodecane, methylcyclopentane, dimethylcyclopentane, cyclohexane,methylcyclohexane, dimethylcyclohexane, cycloheptane, cyclooctane, andcyclononane. Suitable alkenes of 6 to 12 carbon atoms include hexene,heptene, octene, cyclohexene, methylcyclohexene, dimethylcyclohexene,cycloheptene, and cyclooctene. Suitable alkynes of 6 to 12 carbon atomsinclude hexyne, heptyne, and octyne. Suitable alcohols of 3 to 10 carbonatoms include n-propyl alcohol, isopropyl alcohol, 1-butyl alcohol,2-butyl alcohol, isobutyl alcohol, tert-butyl alcohol, 1-pentanol,2-pentanol, 3-pentanol, tert-amyl alcohol, neopentyl alcohol,2-methyl-1-butanol, 3-methyl-1-butanol, 3-methyl-3-pentanol,cyclopentanol, 1-hexanol, 2-hexanol, 3-hexanol, 2,3-dimethyl-2-butanol,3,3-dimethyl-1-butanol, 3,3-dimethyl-2-butanol, 2-ethyl-1-butanol,2-methyl-1-pentanol, 2-methyl-2-pentanol, 2-methyl-3-pentanol,3-methyl-1-pentanol, 3-methyl-2-pentanol, 3-methyl-3-pentanol,4-methyl-1-pentanol, 4-methyl-2-pentanol, 4-methyl-3-pentanol,cyclohexanol, and 1-octanol. Suitable ether compounds of 8 to 12 carbonatoms include di-n-butyl ether, diisobutyl ether, di-sec-butyl ether,di-n-pentyl ether, diisopentyl ether, di-sec-pentyl ether, di-t-amylether, and di-n-hexyl ether. Suitable aromatic solvents include toluene,xylene, ethylbenzene, isopropylbenzene, t-butylbenzene, and mesitylene.The solvents may be used alone or in admixture.

Where a hole pattern is formed by negative tone development, exposure bydouble dipole illuminations of X- and Y-direction line patterns providesthe highest contrast light. The contrast may be further increased bycombining dipole illumination with s-polarized illumination.

In a preferred embodiment, a halftone phase shift mask bearing alattice-like shifter pattern is used, whereby a pattern of holes isformed at the intersections between gratings of the lattice-like shifterpattern after development. More preferably the halftone phase shift maskbearing a lattice-like shifter pattern has a transmittance of 3% to 15%.

In a more preferred embodiment, a phase shift mask including alattice-like first shifter having a line width equal to or less than ahalf pitch and a second shifter arrayed on the first shifter andconsisting of lines whose on-wafer size is 2 to 30 nm thicker than theline width of the first shifter is used, whereby a pattern of holes isformed only where the thick shifter is arrayed. Alternatively, a phaseshift mask including a lattice-like first shifter having a line widthequal to or less than a half pitch and a second shifter arrayed on thefirst shifter and consisting of dots whose on-wafer size is 2 to 100 nmthicker than the line width of the first shifter is used, whereby apattern of holes is formed only where the thick shifter is arrayed.

FIG. 2 is an optical image of X-direction lines having a pitch of 90 nmand a line size of 45 nm printed under conditions: ArF excimer laser ofwavelength 193 nm, NA 1.3 lens, dipole illumination, 6% halftone phaseshift mask, and s-polarization.

FIG. 3 is an optical image of Y-direction lines having a pitch of 90 nmand a line size of 45 nm printed under conditions: ArF excimer laser ofwavelength 193 nm, NA 1.3 lens, dipole illumination, 6% halftone phaseshift mask, and s-polarization. A black area is a light shielded areawhile a white area is a high light intensity area. A definite contrastdifference is recognized between white and black, indicating thepresence of a fully light shielded area.

FIG. 4 shows a contrast image obtained by overlaying the optical imageof X-direction lines in FIG. 2 with that of Y-direction lines in FIG. 3.Against the expectation that a combination of X and Y lines may form alattice-like image, weak light black areas draw circular shapes. As thepattern (circle) size becomes larger, the circular shape changes to arhombic shape to merge with adjacent ones. As the circle size becomessmaller, circularity is improved, which is evidenced by the presence ofa fully light shielded small circle.

Exposure by double dipole illuminations of X- and Y-direction linescombined with polarized illumination presents a method of forming lightof the highest contrast. This method, however, has the drawback that thethroughput is substantially reduced by double exposures and maskexchange therebetween. To continuously carry out two exposures whileexchanging a mask, the exposure apparatus must be equipped with two maskstages although the existing exposure apparatus includes a single maskstage. Higher throughputs may be obtained by carrying out exposure ofX-direction lines continuously on 25 wafers in a front-opening unifiedpod (FOUP), exchanging the mask, and carrying out exposure continuouslyon the same 25 wafers, rather than exchanging a mask on every exposureof a single wafer. However, a problem arises that as the time durationuntil the first one of 25 wafers is exposed in the second exposure isprolonged, the environment affects the resist such that the resist afterdevelopment may change its size and shape. To block the environmentalimpact on wafers in standby until the second exposure, it is effectivethat the resist film is overlaid with a protective film.

To proceed with a single mask, it is proposed in Non-Patent Document 1to carry out two exposures by dipole illuminations in X- andY-directions using a mask bearing a lattice-like pattern. When thismethod is compared with the above method using two masks, the opticalcontrast is somewhat reduced, but the throughput is improved by the useof a single mask. As described in Non-Patent Document 1, the methodinvolves forming X-direction lines in a first photoresist film byX-direction dipole illumination using a mask bearing a lattice-likepattern, insolubilizing the X-direction lines by light irradiation,coating a second photoresist film thereon, and forming Y-direction linesby Y-direction dipole illumination, thereby forming holes at theinterstices between X- and Y-direction lines. Although only a singlemask is needed, this method includes additional steps of insolubilizingthe first photoresist pattern between the two exposures, and coating anddeveloping the second photoresist film. Then the wafer must be removedfrom the exposure stage between the two exposures to give rise to theproblem of an increased alignment error. To minimize the alignment errorbetween two exposures, two exposures must be continuously carried outwithout removing the wafer from the exposure stage. FIG. 21 shows theshape of apertures for dipole illumination for forming X-direction orhorizontal lines using a mask bearing a lattice-like pattern, and FIG.22 shows the shape of apertures for dipole illumination for formingY-direction or vertical lines. The addition of s-polarized illuminationto dipole illumination provides a further improved contrast and is thuspreferably employed. After two exposures for forming X- and Y-directionlines using a lattice-like mask are performed in an overlapping manner,negative tone development is performed whereupon a hole pattern isformed.

When it is desired to form a hole pattern via a single exposure using alattice-like mask, a quadra-pole illumination or cross-pole illuminationin the aperture configuration shown in FIG. 23 is used. The contrast maybe improved by combining it with X-Y polarized illumination orazimuthally polarized illumination of circular polarization.

In the hole pattern forming process of the invention, when two exposuresare involved, these exposures are carried out by changing theillumination and mask for the second exposure from those for the firstexposure, whereby a fine size pattern can be formed at the highestcontrast and to dimensional uniformity. The masks used in the first andsecond exposures bear first and second patterns of intersecting lineswhereby a pattern of holes at intersections of lines is formed in theresist film after development. The first and second lines are preferablyat right angles although an angle of intersection other than 90° may beemployed. The first and second lines may have the same or different sizeand/or pitch. If a single mask bearing first lines in one area andsecond lines in a different area is used, it is possible to performfirst and second exposures continuously. In this case, however, themaximum area available for exposure is one half. Notably, the continuousexposures lead to a minimized alignment error. Of course, the singleexposure provides a smaller alignment error than the two continuousexposures.

When two exposures are performed using a single mask without reducingthe exposure area, the mask pattern may be a lattice-like pattern asshown in FIG. 5, a dot pattern as shown in FIG. 7, or a combination of adot pattern and a lattice-like pattern as shown in FIG. 11. The use of alattice-like pattern contributes to the most improved light contrast,but has the drawback of a reduced resist sensitivity due to a loweringof light intensity. On the other hand, the use of a dot pattern suffersa lowering of light contrast, but provides the merit of an improvedresist sensitivity.

Where holes are arrayed in horizontal and vertical directions, theabove-described illumination and mask pattern are used. Where holes arearrayed at a different angle, for example, at an angle of 45°, a mask ofa 45° arrayed pattern is combined with dipole illumination or cross-poleillumination.

Where two exposures are performed, a first exposure by a combination ofdipole illumination with polarized illumination for enhancing thecontrast of X-direction lines is followed by a second exposure by acombination of dipole illumination with polarized illumination forenhancing the contrast of Y-direction lines. Two continuous exposureswith the X- and Y-direction contrasts emphasized through a single maskcan be performed on a currently commercially available scanner.

The method of combining X and Y polarized illuminations with cross-poleillumination using a mask bearing a lattice-like pattern can form a holepattern through a single exposure, despite a slight lowering of lightcontrast as compared with two exposures of dipole illumination. Themethod is estimated to attain a substantial improvement in throughputand avoids the problem of misalignment between two exposures. Using sucha mask and illumination, a hole pattern of the order of 40 nm can beformed at a practically acceptable cost.

On use of a mask bearing a lattice-like pattern as shown in FIG. 5 wherelight is fully shielded at intersections between gratings, black spotshaving a very high degree of light shielding appear as shown in FIG. 6.FIG. 6 is an optical image of a lattice-like line pattern having a pitchof 90 nm and a line width of 30 nm printed under conditions: NA 1.3lens, cross-pole illumination, 6% halftone phase shift mask, andazimuthally polarized illumination. A fine hole pattern may be formed byperforming exposure through a mask bearing such a pattern and organicsolvent development entailing positive/negative reversal.

On use of a mask bearing a dot pattern of square dots having a pitch of90 nm and a side width of 60 nm as shown in FIG. 7, under conditions: NA1.3 lens, cross-pole illumination, 6% halftone phase shift mask, andazimuthally polarized illumination, an optical image is obtained asshown in FIG. 8 that depicts the contrast thereof. Although the circleof fully light shielded spot in FIG. 8 has a smaller area than in FIG.6, which indicates a low contrast as compared with the lattice-likepattern mask, the formation of a hole pattern is possible owing to thepresence of black or light shielded spots.

It is difficult to form a fine hole pattern that holes are randomlyarrayed at varying pitch and position. The super-resolution technologyusing off-axis illumination (such as dipole or cross-pole illumination)in combination with a phase shift mask and polarization is successful inimproving the contrast of dense (or grouped) patterns, but not so thecontrast of isolated patterns.

When the super-resolution technology is applied to repeating densepatterns, the pattern density bias between dense and isolated patterns,known as proximity bias, becomes a problem. As the super-resolutiontechnology used becomes stronger, the resolution of a dense pattern ismore improved, but the resolution of an isolated pattern remainsunchanged. Then the proximity bias is exaggerated. In particular, anincrease of proximity bias in a hole pattern resulting from furtherminiaturization poses a serious problem. One common approach taken tosuppress the proximity bias is by biasing the size of a mask pattern.Since the proximity bias varies with properties of a photoresistcomposition, specifically dissolution contrast and acid diffusion, theproximity bias of a mask varies with the type of photoresistcomposition. For a particular type of photoresist composition, a maskhaving a different proximity bias must be used. This adds to the burdenof mask manufacturing. Then the pack and unpack (PAU) method is proposedin Proc. SPIE Vol. 5753, p171 (2005), which involves strongsuper-resolution illumination of a first positive resist to resolve adense hole pattern, coating the first positive resist pattern with anegative resist film material in alcohol solvent which does not dissolvethe first positive resist pattern, exposure and development of anunnecessary hole portion to close the corresponding holes, therebyforming both a dense pattern and an isolated pattern. One problem of thePAU method is misalignment between first and second exposures, as theauthors point out in the report. The hole pattern which is not closed bythe second development experiences two developments and thus undergoes asize change, which is another problem.

To form a random pitch hole pattern by organic solvent developmententailing positive/negative reversal, a mask is used in which alattice-like pattern is arrayed over the entire surface and the width ofgratings is thickened only where holes are to be formed. As shown inFIG. 9, on a lattice-like pattern having a pitch of 90 nm and a linewidth of 20 nm, thick crisscross or intersecting line segments aredisposed where dots are to be formed. A black area corresponds to thehalftone shifter portion. Line segments with a width of 30 nm aredisposed in the dense pattern portion whereas thicker line segments(width 40 nm in FIG. 9) are disposed in more isolated pattern portions.Since the isolated pattern provides light with a lower intensity thanthe dense pattern, thicker line segments are used. Since the peripheralarea of the dense pattern provides light with a relatively lowintensity, line segments having a width of 32 nm are assigned to theperipheral area which width is slightly greater than that in theinternal area of the dense pattern. FIG. 10 shows an optical image fromthe mask of FIG. 9, indicating the contrast thereof. Black or lightshielded areas are where holes are formed via positive/negativereversal. Black spots are found at positions other than where holes areformed, but few are transferred in practice because they are of smallsize. Optimization such as reduction of the width of grating linescorresponding to unnecessary holes can inhibit transfer of unnecessaryholes.

Also useful is a mask in which a lattice-like pattern is arrayed overthe entire surface and thick dots are disposed only where holes are tobe formed. As shown in FIG. 11, on a lattice-like pattern having a pitchof 90 nm and a line width of 15 nm, thick dots are disposed where dotsare to be formed. A black area corresponds to the halftone shifterportion. Square dots having one side with a size of 55 nm are disposedin the dense pattern portion whereas larger square dots (side size 90 nmin FIG. 11) are disposed in more isolated pattern portions. Althoughsquare dots are shown in the figure, the dots may have any shapeincluding rectangular, rhombic, pentagonal, hexagonal, heptagonal,octagonal, and polygonal shapes and even circular shape.

FIG. 12 shows an optical image from the mask of FIG. 11, indicating thecontrast thereof. The presence of black or light shielded spotssubstantially equivalent to those of FIG. 10 indicates that holes areformed via positive/negative reversal.

On use of a mask bearing no lattice-like pattern arrayed as shown inFIG. 13, black or light shielded spots do not appear as shown in FIG.14. In this case, holes are difficult to form, or even if holes areformed, a variation of mask size is largely reflected by a variation ofhole size because the optical image has a low contrast.

Example

Examples of the invention are given below by way of illustration and notby way of limitation. The abbreviation “pbw” is parts by weight. Mestands for methyl. For all polymers, Mw and Mn are determined by GPCversus polystyrene standards using tetrahydrofuran solvent. For patternprofile observation, a top-down scanning electron microscope (TDSEM)S-9380 (Hitachi Hitechnologies, Ltd.) was used.

Synthesis Example 1

Acetal compounds within the scope of the invention were synthesized inaccordance with the formulation shown below.

Synthesis Example 1-1 Synthesis of Monomer 1

In 200 ml of acetonitrile, 50.0 g of Starting Monomer 1 was mixed with43.8 g of diisopropylethylamine and 0.05 g of2,2′-methylenebis(6-t-butyl-p-cresol). To the mixture below 20° C., 40.4g of chloromethyl neopentyl ether was added dropwise, followed bystirring at 40° C. for 8 hours. 300 ml of water was added to quench thereaction, followed by standard workup. Purification by distillation gave59.9 g (yield 84%) of the end compound.

Boiling point: 102-104° C./10 Pa

IR (D-ATR): ν=2951, 2916, 2867, 1713, 1637, 1456, 1396, 1362, 1326,1300, 1171, 1121, 1072, 1042, 1011, 982 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆): δ=0.80 (9H, s), 1.47 (2H, t), 1.65-1.75(4H, m), 1.80 (3H, s), 1.93-2.05 (4H, m), 2.11 (2H, s), 2.24-2.28 (2H,m), 3.12 (2H, s), 4.74 (2H, s), 5.58 (1H, m), 5.91 (1H, s) ppm

Synthesis Example 1-2 Synthesis of Monomer 2

Monomer 2 was prepared by the same procedure as Synthesis Example 1-1except that Starting Monomer 2 was used instead of Starting Monomer 1.Yield 86%.

Synthesis Example 1-3 Synthesis of Monomer 3

Monomer 3 was prepared by the same procedure as Synthesis Example 1-1except that chloromethyl neohexyl ether was used instead of chloromethylneopentyl ether. Yield 85%.

Synthesis Example 1-4 Synthesis of Monomer 4

Monomer 4 was prepared by the same procedure as Synthesis Example 1-1except that chloromethyl (2-methyl-2-norbornyl)methyl ether was usedinstead of chloromethyl neopentyl ether. Yield 79%.

Synthesis Example 1-5 Synthesis of Monomer 5

Monomer 5 was prepared by the same procedure as Synthesis Example 1-1except that chloromethyl (1-adamantyl)-methyl ether was used instead ofchloromethyl neopentyl ether. Yield 82%.

Synthesis Example 1-6 Synthesis of Monomer 6

Monomer 6 was prepared by the same procedure as Synthesis Example 1-1except that Starting Monomer 3 was used instead of Starting Monomer 1.Yield 80%.

IR (D-ATR): ν=2953, 2868, 1717, 1637, 1480, 1453, 1396, 1363, 1321,1294, 1242, 1164, 1140, 1080, 1041, 983 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆): δ=0.85 (18H, s), 1.58-1.67 (4H, m), 1.76(2H, d), 1.80 (2H, s), 1.87 (1H, d), 1.93 (2H, m), 2.06-2.13 (4H, m),2.30-2.34 (1H, m), 3.12 (4H, s), 4.74 (4H, s), 5.58 (1H, m), 5.91 (1H,s) ppm

Synthesis Example 1-7 Synthesis of Monomer 7

Monomer 7 was prepared by the same procedure as Synthesis Example 1-1except that Starting Monomer 3 was used instead of Starting Monomer 1,and chloromethyl (2-methyl-2-norbornyl)methyl ether was used instead ofchloromethyl neopentyl ether. Yield 78%.

Synthesis Example 1-8 Synthesis of Monomer 8

Monomer 8 was prepared by the same procedure as Synthesis Example 1-1except that Starting Monomer 3 was used instead of Starting Monomer 1,and chloromethyl (1-adamantyl)-methyl ether was used instead ofchloromethyl neopentyl ether. Yield 74%.

Synthesis Example 1-9 Synthesis of Monomer 9

Monomer 9 was prepared by the same procedure as

Synthesis Example 1-1 except that Starting Monomer 4 was used instead ofStarting Monomer 1. Yield 76%.

Synthesis Example 1-10 Synthesis of Monomer 10

Monomer 10 was prepared by the same procedure as Synthesis Example 1-1except that Starting Monomer 5 was used instead of Starting Monomer 1.Yield 86%.

Boiling point: 122-125° C./10 Pa

IR (D-ATR): ν=2953, 2909, 2855, 1713, 1637, 1454, 1397, 1329, 1313,1301, 1161, 1114, 1064, 1044, 1012, 970 cm⁻¹

¹H-NMR (600 MHz in DMSO-d₆): δ=0.86 (9H, s), 1.38-1.49 (4H, m),1.50-1.63 (2H, m), 1.80 (3H, s), 1.87 (2H, s), 1.96-2.08 (4H, m), 2.18(2H, s), 3.10 (4H, d), 4.58 (2H, s), 5.56 (1H, m), 5.90 (1H, s) ppm

Monomers 1 to 10 obtained in Synthesis Example 1 have the structuralformulae shown below.

Synthesis Example 2

Various polymers (Polymers 1 to 29 and Comparative Polymers 1 to 3) foruse in resist compositions were prepared by combining suitable monomers,effecting copolymerization reaction in tetrahydrofuran solvent, pouringinto methanol for crystallization, repeatedly washing with hexane,isolation, and drying. The polymers were analyzed by ¹H-NMR to determinetheir composition and by GPC to determine Mw and dispersity Mw/Mn.

Preparation of Positive Resist Composition and Alkali-Soluble ProtectiveFilm-Forming Composition

A resist composition in solution form was prepared by dissolvingpolymers (Resist Polymer) and components in solvents in accordance withthe formulation of Tables 2 and 3. A protective film-forming compositionin solution form was prepared by dissolving polymers (TC Polymer) andcomponents in solvents in accordance with the formulation of Table 4.The solutions were filtered through a Teflon® filter with a pore size of0.2 μm. The components are identified below.

-   Acid generator: PAG1 to PAG5 of the following structural formulae

-   Basic Compound: Quenchers 1 and 2 of the following structural    formulae-   Organic Solvent: PGMEA (propylene glycol monomethyl ether acetate)    -   CyH (cyclohexanone)

ArF Lithography Patterning Test 1

A resist composition was prepared by dissolving polymers (ResistPolymer) and components in solvents in accordance with the formulationof Table 1. On a substrate (silicon wafer) having an antireflectivecoating (Nissan Chemical Industry Co., Ltd.) of 80 nm thick, the resistcomposition was spin coated and baked on a hot plate at 100° C. for 60seconds to form a resist film of 160 nm thick.

Using an ArF excimer laser scanner NSR-305B (Nikon Corp., NA 0.68,σ0.73), the resist film was open-frame exposed in a dose which variedstepwise by 0.2 mJ/cm². The exposed resist film was baked (PEB) at 110°C. for 60 seconds and puddle developed for 60 seconds with an organicsolvent developer as shown in Table 1. The wafer was rinsed at 500 rpmwith a rinse liquid (organic solvent) as shown in Table 1, spin dried at2,000 rpm, and baked at 100° C. for 60 seconds to evaporate off therinse liquid. Separately, the same process was repeated until the PEB,and followed by development with a 2.38 wt % tetramethylammoniumhydroxide (TMAH) aqueous solution. The film thickness after PEB, thefilm thickness after organic solvent development (butyl acetate BA), andthe film thickness after TMAH aqueous solution development weremeasured. A contrast curve was determined by plotting the film thicknessversus the exposure dose. The results are shown in FIGS. 15 to 17.

TABLE 1 Acid Basic Organic Polymer generator compound solvent Rinse(pbw) (pbw) (pbw) (pbw) Developer liquid Example 1-1 Resist 1-1 Polymer1 PAG1 Quencher 1 PGMEA butyl 4-methyl-2- (100) (6.5) (1.50) (800)acetate pentanol CyH (400) Comparative Comparative Comparative PAG1Quencher 1 PGMEA butyl 4-methyl-2- Example 1-1 Resist 1-1 Polymer 1(6.5) (1.50) (800) acetate pentanol (100) CyH (400) ComparativeComparative Comparative PAG1 Quencher 1 PGMEA butyl 4-methyl-2- Example1-2 Resist 1-2 Polymer 2 (6.5) (1.50) (800) acetate pentanol (100) CyH(400)

ArF Lithography Patterning Test 2

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A941 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition shown in Table 4 was spin coatedon the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick. In Examples 2-19 to 2-33and Comparative Example 2-4, the protective film was omitted.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, cross-pole opening 20 deg., azimuthallypolarized illumination), exposure was performed in a varying dosethrough a 6% halftone phase shift mask bearing a lattice-like patternwith a pitch of 90 nm and a line width of 30 nm (on-wafer size) whoselayout is shown in FIG. 18. After the exposure, the wafer was baked(PEB) at the temperature shown in Table 5 for 60 seconds and developed.Specifically, butyl acetate was injected from a development nozzle whilethe wafer was spun at 30 rpm for 3 seconds, which was followed bystationary puddle development for 27 seconds. The wafer was rinsed withdiisoamyl ether, spin dried, and baked at 100° C. for 20 seconds toevaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of 50 holes was measured, fromwhich a size variation 3σ was determined. The results are shown in Table5.

TABLE 2 Acid Basic Organic Polymer generator compound Additive solvent(pbw) (pbw) (pbw) (pbw) (pbw) Resist 2-1 Polymer 1 PAG2 Quencher 1 —PGMEA (2,000) (100) (12.5) (1.50) CyH (500) Resist 2-2 Polymer 2 PAG2Quencher 1 — PGMEA (2,000) (100) (12.5) (1.50) CyH (500) Resist 2-3Polymer 3 PAG2 Quencher 1 — PGMEA (2,000) (100) (12.5) (1.50) CyH (500)Resist 2-4 Polymer 4 PAG2 Quencher 1 — PGMEA (2,000) (100) (12.5) (1.50)CyH (500) Resist 2-5 Polymer 5 PAG2 Quencher 1 — PGMEA (2,000) (100)(12.5) (1.50) CyH (500) Resist 2-6 Polymer 6 PAG2 Quencher 1 — PGMEA(2,000) (100) (12.5) (1.50) CyH (500) Resist 2-7 Polymer 7 PAG2 Quencher1 — PGMEA (2,000) (100) (12.5) (1.50) CyH (500) Resist 2-8 Polymer 8PAG2 Quencher 1 — PGMEA (2,000) (100) (12.5) (1.50) CyH (500) Resist 2-9Polymer 9 PAG2 Quencher 1 — PGMEA (2,000) (100) (12.5) (1.50) CyH (500)Resist 2-10 Polymer 10 PAG2 Quencher 1 — PGMEA (2,000) (100) (12.5)(1.50) CyH (500) Resist 2-11 Polymer 11 PAG2 Quencher 1 — PGMEA (2,000)(100) (12.5) (1.50) CyH (500) Resist 2-12 Polymer 12 PAG2 Quencher 1 —PGMEA (2,000) (100) (12.5) (1.50) CyH (500) Resist 2-13 Polymer 13 PAG2Quencher 1 — PGMEA (2,000) (100) (12.5) (1.50) CyH (500) Resist 2-14Polymer 14 PAG2 Quencher 1 — PGMEA (2,000) (100) (12.5) (1.50) CyH (500)Resist 2-15 Polymer 12 PAG2 Quencher 1 Water-repellent Polymer 1 PGMEA(2,000) (100) (12.5) (1.50) (6) CyH (500) Resist 2-16 Polymer 12 PAG2Quencher 1 Water-repellent Polymer 2 PGMEA (2,000) (100) (12.5) (1.50)(6) CyH (500) Resist 2-17 Polymer 12 PAG2 Quencher 1 Water-repellentPolymer 3 PGMEA (2,000) (100) (12.5) (1.50) (6) CyH (500) Resist 2-18Polymer 12 PAG2 Quencher 1 Water-repellent Polymer 4 PGMEA (2,000) (100)(12.5) (1.50) (6) CyH (500) Resist 2-19 Polymer 15 PAG2 Quencher 2Water-repellent Polymer 4 PGMEA (2,400) (100) (12.5) (1.40) (6) ethyleneglycol (100) Resist 2-20 Polymer 16 PAG2 Quencher 2 Water-repellentPolymer 4 PGMEA (2,400) (100) (12.5) (1.40) (6) propylene glycol (100)Resist 2-21 Polymer 17 PAG2 Quencher 2 Water-repellent Polymer 4 PGMEA(2,000) (100) (12.5) (1.40) (6) CyH (500) Resist 2-22 Polymer 18 PAG2Quencher 2 Water-repellent Polymer 4 PGMEA (2,000) (100) (12.5) (1.40)(6) CyH (500) Resist 2-23 Polymer 19 PAG2 Quencher 2 Water-repellentPolymer 4 PGMEA (2,000) (100) (12.5) (1.40) (6) CyH (500) Resist 2-24Polymer 20 PAG3 (3.5) — Water-repellent Polymer 4 PGMEA (2,000) (100)PAG5 (4.5) (6) CyH (500) Resist 2-25 Polymer 21 PAG4 (3.5) Quencher 2Water-repellent Polymer 4 PGMEA (2,000) (100) PAG5 (4.5) (1.40) (6) CyH(500) Resist 2-26 Polymer 22 PAG3 Quencher 2 Water-repellent Polymer 4PGMEA (2,000) (100) (12.5) (1.40) (6) CyH (500) Resist 2-27 Polymer 23PAG4 Quencher 2 Water-repellent Polymer 4 PGMEA (2,000) (100) (12.5)(1.40) (6) CyH (500) Resist 2-28 Polymer 24 PAG3 Quencher 2Water-repellent Polymer 4 PGMEA (2,000) (1005 (12.5) (1.40) (6) CyH(500) Resist 2-29 Polymer 25 PAG3 Quencher 2 Water-repellent Polymer 4PGMEA (2,000) (100) (12.5) (1.40) (6) CyH (500)

TABLE 3 Acid Basic Organic Polymer generator compound Additive solvent(pbw) (pbw) (pbw) (pbw) (pbw) Resist 2-30 Polymer 26 PAG3 Quencher 2Water-repellent Polymer 4 PGMEA (2,000) (100) (12.5) (1.40) (6) CyH(500) Resist 2-31 Polymer 27 PAG3 Quencher 2 Water-repellent Polymer 4PGMEA (2,000) (100) (12.5) (1.40) (6) CyH (500) Resist 2-32 Polymer 28PAG3 Quencher 2 Water-repellent Polymer 4 PGMEA (2,000) (100) (12.5)(1.40) (6) CyH (500) Resist 2-33 Polymer 29 PAG3 Quencher 2Water-repellent Polymer 4 PGMEA (2,000) (50) (12.5) (1.40) (6) CyH (400)Comparative propylene glycol Polymer 1 (100) (50) Resist 2-34 Polymer 29PAG3 Quencher 2 Water-repellent Polymer 4 PGMEA (2,000) (50) (12.5)(1.40) (6) CyH (500) Polymer 1 (50) Comparative Comparative PAG2Quencher 1 — PGMEA (2,000) Resist 2-1 Polymer 1 (12.5) (1.50) CyH (500)(100) Comparative Comparative PAG2 Quencher 1 — PGMEA (2,000) Resist 2-2Polymer 2 (12.5) (1.50) CyH (500) (100) Comparative Comparative PAG2Quencher 1 — PGMEA (2,000) Resist 2-3 Polymer 3 (12.5) (1.50) CyH (500)(100) Comparative Comparative PAG2 Quencher 1 Water-repellent Polymer 5PGMEA (2,000) Resist 2-4 Polymer 3 (12.5) (1.50) (6) CyH (500) (100)

TABLE 4 Protective Polymer Additive Organic solvent Film (pbw) (pbw)(pbw) TC-1 TC Polymer 1 tri-n- diisoamyl ether (2,700) (100) octylamine2-methyl-1-butanol (270) (0.5) TC-2 TC Polymer 2 tri-n- diisoamyl ether(2,700) (100) octylamine 2-methyl-1-butanol (270) (0.5) TC-3 TC Polymer3 — diisoamyl ether (2,700) (100) 2-methyl-1-butanol (270) TC-4 TCPolymer 2 — diisoamyl ether (2,700) (80) 2-methyl-1-butanol (270) TCPolymer 4 (20) TC-5 TC Polymer 5 — diisoamyl ether (2,700) (80)2-methyl-1-butanol (270) TC Polymer 4 (20) Comparative TC Polymer 1 —diisoamyl ether (2,700) TC-1 (100) 2-methyl-1-butanol (270) ComparativeTC Polymer 6 — diisoamyl ether (2,700) TC-2 (100) 2-methyl-1-butanol(270)

TABLE 5 Hole size PEB variation Protective temp. Dose 3σ Resist film (°C.) (mJ/cm²) (nm) Example 2-1 Resist 2-1 TC-1 105 48 2.0 Example 2-2Resist 2-2 TC-1 95 33 2.0 Example 2-3 Resist 2-3 TC-1 100 38 1.9 Example2-4 Resist 2-4 TC-1 100 38 2.1 Example 2-5 Resist 2-5 TC-1 95 34 2.2Example 2-6 Resist 2-6 TC-1 100 46 2.2 Example 2-7 Resist 2-7 TC-1 10038 2.2 Example 2-8 Resist 2-8 TC-1 110 39 2.4 Example 2-9 Resist 2-9TC-1 90 34 2.2 Example 2-10 Resist 2-10 TC-1 90 30 2.1 Example 2-11Resist 2-11 TC-1 115 44 2.1 Example 2-12 Resist 2-12 TC-1 100 42 2.3Example 2-13 Resist 2-12 TC-2 100 42 2.4 Example 2-14 Resist 2-12 TC-3100 42 2.1 Example 2-15 Resist 2-12 TC-4 100 41 2.5 Example 2-16 Resist2-12 TC-5 100 41 2.6 Example 2-17 Resist 2-13 TC-1 115 47 2.6 Example2-18 Resist 2-14 TC-1 105 46 2.6 Example 2-19 Resist 2-15 — 130 68 4.3Example 2-20 Resist 2-16 — 130 72 4.1 Example 2-21 Resist 2-17 — 100 433.5 Example 2-22 Resist 2-18 — 100 43 3.4 Example 2-23 Resist 2-19 — 9547 3.8 Example 2-24 Resist 2-20 — 95 49 2.8 Example 2-25 Resist 2-21 —100 47 2.7 Example 2-26 Resist 2-22 — 90 42 2.0 Example 2-27 Resist 2-23— 110 49 3.0 Example 2-28 Resist 2-24 — 100 47 2.7 Example 2-29 Resist2-25 — 90 42 2.3 Example 2-30 Resist 2-26 — 100 47 2.7 Example 2-31Resist 2-27 — 100 42 2.3 Example 2-32 Resist 2-28 — 105 42 2.3 Example2-33 Resist 2-29 — 105 42 2.3 Example 2-34 Resist 2-30 — 100 42 2.3Example 2-35 Resist 2-31 — 100 47 2.7 Example 2-36 Resist 2-32 — 100 422.3 Example 2-37 Resist 2-33 — 100 42 3.2 Example 2-38 Resist 2-34 — 10042 2.8

ArF Lithography Patterning Test 3

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, cross-pole opening 20 deg., azimuthallypolarized illumination), exposure was performed through a 6% halftonephase shift mask bearing a lattice-like pattern with a pitch of 90 nmand a line width of 15 nm (on-wafer size) having dots disposed atintersections, whose layout is shown in FIG. 19, while the dose andfocus were varied. After the exposure, the wafer was baked (PEB) at thetemperature shown in Table 6 for 60 seconds and developed. Specifically,butyl acetate was injected from a development nozzle while the wafer wasspun at 30 rpm for 3 seconds, which was followed by stationary puddledevelopment for 27 seconds. The wafer was rinsed with diisoamyl ether,spin dried, and baked at 100° C. for 20 seconds to evaporate off therinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of 50 holes was measured, fromwhich a size variation 3σ was determined. The results are shown in Table6.

TABLE 6 Hole size variation PEB temp. Dose 3σ Resist (° C.) (mJ/cm²)(nm) Example 3-1 Resist 2-1 105 52 1.9 Example 3-2 Resist 2-2 95 41 1.9Comparative Comparative 110 115 4.6 Example 3-1 Resist 2-1 ComparativeComparative 105 105 5.0 Example 3-2 Resist 2-2

Arf Lithography Patterning Test 4

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, cross-pole opening 20 deg., azimuthallypolarized illumination), exposure was performed through a 6% halftonephase shift mask bearing a lattice-like pattern with a pitch of 90 nm(on-wafer size) having thick gratings disposed at intersections whoselayout is shown in FIG. 20, while the dose was varied. After theexposure, the wafer was baked (PEB) at the temperature shown in Table 7for 60 seconds and developed. Specifically, butyl acetate was injectedfrom a development nozzle while the wafer was spun at 30 rpm for 3seconds, which was followed by stationary puddle development for 27seconds. The wafer was rinsed with diisoamyl ether, spin dried, andbaked at 100° C. for 20 seconds to evaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of holes at positions A and Bon the mask (FIG. 20) was measured. The results are shown in Table 7.

TABLE 7 PEB Hole size Hole size temp. Dose at A at B (° C.) (mJ/cm²)(nm) (nm) Example 4-1 Resist 2-1 105 53 40 41 Example 4-2 Resist 2-2 9543 39 41 Comparative Comparative 110 85 23 51 Example 4-1 Resist 2-1Comparative Comparative 105 80 21 50 Example 4-2 Resist 2-2

ArF Lithography Patterning Test 5

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A941 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, dipole opening 20 deg., azimuthallypolarized illumination), exposure was performed through a 6% halftonephase shift mask bearing a lattice-like pattern with a pitch of 90 nmand a line width of 30 nm (on-wafer size) whose layout is shown in FIG.18 while the dose was varied. The same area was subjected to twocontinuous exposures by X and Y dipole illuminations. After theexposure, the wafer was baked (PEB) at the temperature shown in Table 8for 60 seconds and developed. Specifically, butyl acetate was injectedfrom a development nozzle while the wafer was spun at 30 rpm for 3seconds, which was followed by stationary puddle development for 27seconds. The wafer was rinsed with diisoamyl ether, spin dried, andbaked at 100° C. for 20 seconds to evaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of 50 holes was measured, fromwhich a size variation 3σ was determined. The results are shown in Table8.

TABLE 8 Hole size variation PEB temp. Dose 3σ Resist (° C.) (mJ/cm²)(nm) Example 5-1 Resist 2-1 105 15 1.8 Example 5-2 Resist 2-2 95 18 1.9Comparative Comparative 110 22 3.1 Example 5-1 Resist 2-1 ComparativeComparative 105 20 3.2 Example 5-2 Resist 2-2

ArF Lithography Patterning Test 6

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, dipole opening 20 deg., azimuthallypolarized illumination), exposure was performed through a 6% halftonephase shift mask bearing a lattice-like pattern with a pitch of 90 nmand a line width of 15 nm (on-wafer size) having dots disposed atintersections, whose layout is shown in FIG. 19, while the dose wasvaried. The same area was subjected to two continuous exposures by X andY dipole illuminations. After the exposure, the wafer was baked (PEB) atthe temperature shown in Table 9 for 60 seconds and developed.Specifically, butyl acetate was injected from a development nozzle whilethe wafer was spun at 30 rpm for 3 seconds, which was followed bystationary puddle development for 27 seconds. The wafer was rinsed withdiisoamyl ether, spin dried, and baked at 100° C. for 20 seconds toevaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of holes was measured, fromwhich a focus margin affording a size of 40 nm±5 nm was determined asDOF. The size of 50 holes within a shot of the same dose and the samefocus was measured, from which a size variation 3σ was determined. Theresults are shown in Table 9.

TABLE 9 PEB Hole size temp. Dose DOF variation 3σ Resist (° C.) (mJ/cm²)(nm) (nm) Example 6-1 Resist 2-1 105 22 110 2.2 Example 6-2 Resist 2-295 24 100 2.1 Comparative Comparative 110 33 30 3.6 Example 6-1 Resist2-1 Comparative Comparative 105 35 20 3.0 Example 6-2 Resist 2-2

ArF Lithography Patterning Test 7

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, cross-pole opening 20 deg., azimuthallypolarized illumination), exposure was performed through a 6% halftonephase shift mask bearing a dot pattern with a pitch of 90 nm and a widthof 55 nm (on-wafer size) whose layout is shown in FIG. 7, while the dosewas varied. After the exposure, the wafer was baked (PEB) at thetemperature shown in Table 10 for 60 seconds and developed.Specifically, butyl acetate was injected from a development nozzle whilethe wafer was spun at 30 rpm for 3 seconds, which was followed bystationary puddle development for 27 seconds. The wafer was rinsed withdiisoamyl ether, spin dried, and baked at 100° C. for 20 seconds toevaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of holes was measured, fromwhich a focus margin affording a size of 40 nm±5 nm was determined asDOF. The size of 50 holes within a shot of the same dose and the samefocus was measured, from which a size variation 3σ was determined. Theresults are shown in Table 10.

TABLE 10 PEB Hole size temp. Dose DOF variation 3σ Resist (° C.)(mJ/cm²) (nm) (nm) Example 7-1 Resist 2-1 105 21 100 3.2 Example 7-2Resist 2-2 95 29 95 3.1 Comparative Comparative 110 33 15 5.6 Example7-1 Resist 2-1 Comparative Comparative 105 34 10 5.0 Example 7-2 Resist2-2

ArF Lithography Patterning Test 8

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, dipole opening 20 deg., azimuthallypolarized illumination), exposure was performed through a 6% halftonephase shift mask bearing a dot pattern with a pitch of 90 nm and a widthof 55 nm (on-wafer size) whose layout is shown in FIG. 7, while the dosewas varied. The same area was subjected to two continuous exposures by Xand Y dipole illuminations. After the exposure, the wafer was baked(PEB) at the temperature shown in Table 11 for 60 seconds and developed.Specifically, butyl acetate was injected from a development nozzle whilethe wafer was spun at 30 rpm for 3 seconds, which was followed bystationary puddle development for 27 seconds. The wafer was rinsed withdiisoamyl ether, spin dried, and baked at 100° C. for 20 seconds toevaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of holes was measured, fromwhich a focus margin affording a size of 40 nm±5 nm was determined asDOF. The size of 50 holes within a shot of the same dose and the samefocus was measured, from which a size variation 3σ was determined. Theresults are shown in Table 11.

TABLE 11 PEB Hole size temp. Dose DOF variation 3σ Resist (° C.)(mJ/cm²) (nm) (nm) Example 8-1 Resist 2-1 105 22 105 2.0 Example 8-2Resist 2-2 95 24 100 2.0 Comparative Comparative 110 33 20 3.4 Example8-1 Resist 2-1 Comparative Comparative 105 35 15 2.9 Example 8-2 Resist2-2

ArF Lithography Patterning Test 9

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Table 2 was spin coated, then baked on a hot plateat 100° C. for 60 seconds to form a resist film of 100 nm thick. Theprotective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, dipole opening 20 deg., azimuthallypolarized illumination), exposure was performed through a 6% halftonephase shift mask bearing a dot pattern with a pitch of 90 nm and a widthof 55 nm (on-wafer size) whose layout is shown in FIG. 7, while the dosewas varied. The same area was subjected to two continuous exposures by Xand Y dipole illuminations. After the exposure, the wafer was baked(PEB) at the temperature shown in Table 12 for 60 seconds and developed.Specifically, the solvent shown in Table 12 was injected from adevelopment nozzle while the wafer was spun at 30 rpm for 3 seconds,which was followed by stationary puddle development for 27 seconds. Thewafer was rinsed with diisoamyl ether, spin dried, and baked at 100° C.for 20 seconds to evaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of holes was measured, fromwhich a focus margin affording a size of 40 nm±5 nm was determined asDOF. The size of 50 holes within a shot of the same dose and the samefocus was measured, from which a size variation 3σ was determined. Theresults are shown in Table 12.

TABLE 12 PEB Hole size temp. Dose DOF variation Resist (° C.) (mJ/cm²)Developer (nm) 3σ (nm) Example 9-1 Resist 2-1 105 22 2-heptanone 105 2.0Example 9-2 Resist 2-1 105 23 methyl benzoate 110 2.3 Example 9-3 Resist2-1 105 21 ethyl benzoate 105 2.0 Example 9-4 Resist 2-1 105 26 phenylacetate 100 2.2 Example 9-5 Resist 2-1 105 23 benzyl acetate 100 2.2Example 9-6 Resist 2-1 105 23 methyl phenylacetate 100 2.4 Example 9-7Resist 2-1 105 24 methyl benzoate:butyl 100 2.5 acetate = 6:4 Example9-8 Resist 2-1 105 23 methyl benzoate:2- 100 2.3 heptanone = 5:5

ArF Lithography Patterning Test 10

On a substrate (silicon wafer), a spin-on carbon film ODL-50 (Shin-EtsuChemical Co., Ltd.) having a carbon content of 80 wt % was deposited toa thickness of 200 nm and a silicon-containing spin-on hard maskSHB-A940 having a silicon content of 43 wt % was deposited thereon to athickness of 35 nm. On this substrate for trilayer process, the resistcomposition shown in Tables 2 and 3 was spin coated, then baked on a hotplate at 100° C. for 60 seconds to form a resist film of 100 nm thick.The protective film-forming composition TC-1 shown in Table 4 was spincoated on the resist film and baked at 90° C. for 60 seconds to form aprotective film (or topcoat) of 50 nm thick.

Using an ArF excimer laser immersion lithography scanner NSR-610C (NikonCorp., NA 1.30, σ0.98/0.78, dipole opening 20 deg., azimuthallypolarized illumination), first exposure was performed through a 6%halftone phase shift mask bearing an array of X-direction lines with apitch of 80 nm and a line width of 40 nm (on-wafer size) by compliantdipole illumination. Next, second exposure was performed through a 6%halftone phase shift mask bearing an array of Y-direction lines with apitch of 80 nm and a line width of 40 nm (on-wafer size) by compliantdipole illumination. After the exposure, the wafer was baked (PEB) atthe temperature shown in Table 13 for 60 seconds and developed.Specifically, butyl acetate was injected from a development nozzle whilethe wafer was spun at 30 rpm for 3 seconds, which was followed bystationary puddle development for 27 seconds. The wafer was rinsed withdiisoamyl ether, spin dried, and baked at 100° C. for 20 seconds toevaporate off the rinse liquid.

A hole pattern resulted from image reversal by solvent development. Byobservation under TDSEM S-9380, the size of 50 holes was measured, fromwhich a size variation 3σ was determined. The results are shown in Table13.

TABLE 13 PEB Hole size variation temp. Dose 3σ Resist (° C.) (mJ/cm²)(nm) Example 10-1 Resist 2-1 105 236 1.7 Example 10-2 Resist 2-2 95 211.9 Comparative Comparative 110 115 4.7 Example 10-1 Resist 2-1Comparative Comparative 105 105 5.1 Example 10-2 Resist 2-2

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention.Therefore, it is intended that the invention not be limited to theparticular embodiment disclosed as the best mode contemplated forcarrying out this invention, but that the invention will include allembodiments falling within the scope of the appended claims.

Japanese Patent Application No. 2010-067754 is incorporated herein byreference.

Although some preferred embodiments have been described, manymodifications and variations may be made thereto in light of the aboveteachings. It is therefore to be understood that the invention may bepracticed otherwise than as specifically described without departingfrom the scope of the appended claims.

1. A pattern forming process comprising the steps of applying a resistcomposition comprising a polymer comprising recurring units having anacid labile group-substituted hydroxyl group, an acid generator, and anorganic solvent onto a substrate, heat treating the composition to forma resist film, exposing the resist film to high-energy radiation todefine exposed and unexposed regions, heat treating, and developing theexposed film with an organic solvent developer to form a negativepattern wherein the unexposed region of film is dissolved and theexposed region of film is not dissolved.
 2. The process of claim 1wherein said polymer comprises recurring units (a1) and/or (a2)represented by the general formula (1):

wherein R¹ and R⁴ are each independently hydrogen or methyl, R² is astraight, branched or cyclic, di- to penta-valent aliphatic hydrocarbongroup of 1 to 16 carbon atoms which may contain an ether or esterradical, R³ and R⁵ each are an acid labile group, with the proviso thatwhen R² contains an adamantane ring, an acid labile group of acetal formhaving the general formula (2):

wherein R⁶ is a straight, branched or cyclic, monovalent hydrocarbongroup of 1 to 10 carbon atoms, R⁷ and R⁸ are each independently hydrogenor a straight, branched or cyclic, monovalent hydrocarbon group of 1 to10 carbon atoms, R⁷ and R⁸ may bond together to form an aliphatichydrocarbon ring with the carbon atom to which they are attached, and R⁹is a straight, branched or cyclic, monovalent hydrocarbon group of 1 to15 carbon atoms is excluded from R³, m is an integer of 1 to 4, a1 anda2 are numbers in the range: 0≦a1<1.0, 0≦a2<1.0, and 0<a1+a2<1.0.
 3. Theprocess of claim 1 wherein the developer comprises at least one solventselected from the group consisting of 2-octanone, 2-nonanone,2-heptanone, 3-heptanone, 4-heptanone, 2-hexanone, 3-hexanone,diisobutyl ketone, methylcyclohexanone, acetophenone,methylacetophenone, propyl acetate, butyl acetate, isobutyl acetate,amyl acetate, butenyl acetate, isoamyl acetate, phenyl acetate, propylformate, butyl formate, isobutyl formate, amyl formate, isoamyl formate,methyl valerate, methyl pentenoate, methyl crotonate, ethyl crotonate,methyl lactate, ethyl lactate, propyl lactate, butyl lactate, isobutyllactate, amyl lactate, isoamyl lactate, methyl 2-hydroxyisobutyrate,ethyl 2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenylacetate, benzyl acetate, methyl phenylacetate, benzyl formate,phenylethyl formate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate.
 4. The process of claim 1wherein the step of exposing the resist film to high-energy radiationincludes ArF immersion lithography of 193 nm wavelength or EUVlithography of 13.5 nm wavelength.
 5. The process of claim 4 wherein theArF immersion lithography of 193 nm wavelength uses a halftone phaseshift mask bearing a dot shifter pattern, whereby a pattern of holes isformed at the dots after development.
 6. The process of claim 1 whereinthe exposure step uses halftone phase shift masks and includes twoexposures of two intersecting sets of lines, whereby a pattern of holesis formed at the intersections between lines after development.
 7. Theprocess of claim 1 wherein the exposure step uses a halftone phase shiftmask bearing a lattice-like shifter pattern, whereby a pattern of holesis formed at the intersections between gratings of the lattice-likeshifter pattern after development.
 8. The process of claim 5 wherein thehalftone phase shift mask bearing a dot shifter pattern, twointersecting sets of lines, or lattice-like shifter pattern has atransmittance of 3 to 15%.
 9. The process of claim 5 wherein the phaseshift mask used is a phase shift mask including a lattice-like firstshifter having a line width equal to or less than a half pitch and asecond shifter arrayed on the first shifter and consisting of lineswhose on-wafer size is 2 to 30 nm thicker than the line width of thefirst shifter, whereby a pattern of holes is formed only where the thickshifter is arrayed.
 10. The process of claim 5 wherein the phase shiftmask used is a phase shift mask including a lattice-like first shifterhaving a line width equal to or less than a half pitch and a secondshifter arrayed on the first shifter and consisting of dots whoseon-wafer size is 2 to 100 nm thicker than the line width of the firstshifter, whereby a pattern of holes is formed only where the thickshifter is arrayed.
 11. The pattern forming process of claim 1,comprising the steps of applying a resist composition comprising apolymer comprising recurring units having an acid labilegroup-substituted hydroxyl group, an acid generator, and an organicsolvent onto a substrate, heat treating the composition to form a resistfilm, forming a protective film on the resist film, exposing the resistfilm to high-energy radiation to define exposed and unexposed regions,heat treating, and applying a developer to the coated substrate to forma negative pattern wherein the unexposed region of resist film and theprotective film are dissolved and the exposed region of resist film isnot dissolved.
 12. The process of claim 11 wherein the protective filmis formed of a composition comprising a polymer bearing a1,1,1,3,3,3-hexafluoro-2-propanol residue and an amino group or aminesalt-containing compound, or a composition comprising a polymer bearinga 1,1,1,3,3,3-hexafluoro-2-propanol residue and having amino group oramine salt-containing recurring units copolymerized, the compositionfurther comprising an alcohol solvent of at least 4 carbon atoms, anether solvent of 8 to 12 carbon atoms, or a mixture thereof.
 13. Aresist composition comprising a polymer, an acid generator, and anorganic solvent, said polymer being dissolvable in a developer, adaptedto form a negative pattern, and comprising recurring units (a1) and/or(a2) having an acid labile group-substituted hydroxyl group representedby the general formula (1) shown below, and recurring units derived froma monomer having an adhesive group selected from the group consisting ofhydroxyl, cyano, carbonyl, ester, ether, lactone, carboxyl, andcarboxylic anhydride, said developer being selected from the groupconsisting of 2-octanone, 2-nonanone, 2-heptanone, 3-heptanone,4-heptanone, 2-hexanone, 3-hexanone, diisobutyl ketone,methylcyclohexanone, acetophenone, methylacetophenone, propyl acetate,butyl acetate, isobutyl acetate, amyl acetate, butenyl acetate, isoamylacetate, phenyl acetate, propyl formate, butyl formate, isobutylformate, amyl formate, isoamyl formate, methyl valerate, methylpentenoate, methyl crotonate, ethyl crotonate, methyl lactate, ethyllactate, propyl lactate, butyl lactate, isobutyl lactate, amyl lactate,isoamyl lactate, methyl 2-hydroxyisobutyrate, ethyl2-hydroxyisobutyrate, methyl benzoate, ethyl benzoate, phenyl acetate,benzyl acetate, methyl phenylacetate, benzyl formate, phenylethylformate, methyl 3-phenylpropionate, benzyl propionate, ethylphenylacetate, and 2-phenylethyl acetate,

wherein R¹ and R⁴ are each independently hydrogen or methyl, R² is astraight, branched or cyclic, di- to penta-valent aliphatic hydrocarbongroup of 1 to 16 carbon atoms which may contain an ether or esterradical, R³ and R⁵ each are an acid labile group, with the proviso thatwhen R² contains an adamantane ring, an acid labile group of acetal formhaving the general formula (2):

wherein R⁶ is a straight, branched or cyclic, monovalent hydrocarbongroup of 1 to 10 carbon atoms, R⁷ and R⁸ are each independently hydrogenor a straight, branched or cyclic, monovalent hydrocarbon group of 1 to10 carbon atoms, R⁷ and R⁸ may bond together to form an aliphatichydrocarbon ring with the carbon atom to which they are attached, and R⁹is a straight, branched or cyclic, monovalent hydrocarbon group of 1 to15 carbon atoms is excluded from R³, m is an integer of 1 to 4, a1 anda2 are numbers in the range: 0≦a1<1.0, 0≦a2<1.0, and 0<a1+a2<1.0. 14.The resist composition of claim 13 wherein in recurring unit (a1) informula (1), R² contains an adamantane ring.
 15. An acetal compoundhaving the general formula (3):

wherein R¹⁰ is hydrogen or methyl, R¹¹ is a straight, branched orcyclic, tertiary hydrocarbon group of 4 to 20 carbon atoms, R¹² is asingle bond or methylene, and k¹ is an integer of 1 to 3.